Update to latest Snappy to fix compilation issue on latest MacOS XCode (#6496)

Summary:
* **macOS version:** 10.15.2 (Catalina)
* **XCode/Clang version:** Apple clang version 11.0.0 (clang-1100.0.33.16)

Before this bugfix the error generated is:

```
In file included from ./util/compression.h:23:
./snappy-1.1.7/snappy.h:76:59: error: unknown type name 'string'; did you mean 'std::string'?
  size_t Compress(const char* input, size_t input_length, string* output);
                                                          ^~~~~~
                                                          std::string
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/iosfwd:211:65: note: 'std::string' declared here
typedef basic_string<char, char_traits<char>, allocator<char> > string;
                                                                ^
In file included from db/builder.cc:10:
In file included from ./db/builder.h:12:
In file included from ./db/range_tombstone_fragmenter.h:15:
In file included from ./db/pinned_iterators_manager.h:12:
In file included from ./table/internal_iterator.h:13:
In file included from ./table/format.h:25:
In file included from ./options/cf_options.h:14:
In file included from ./util/compression.h:23:
./snappy-1.1.7/snappy.h:85:19: error: unknown type name 'string'; did you mean 'std::string'?
                  string* uncompressed);
                  ^~~~~~
                  std::string
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/iosfwd:211:65: note: 'std::string' declared here
typedef basic_string<char, char_traits<char>, allocator<char> > string;
                                                                ^
2 errors generated.
make: *** [jls/db/builder.o] Error 1
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6496

Differential Revision: D20389254

Pulled By: pdillinger

fbshipit-source-id: 2864245c8d0dba7b2ab81294241a62f2adf02e20

cmake: add option WITH_CORE_TOOLS to exclude tools except ldb and sst_dump (#6506)

Summary:
ldb and sst_dump are most important tools and they don't dependend on gflags. In cmake, we don't have an way to only build these two tools and exclude other tools. This is inconvenient if the environment has a problem with gflags. Add such an option WITH_CORE_TOOLS.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6506

Test Plan: cmake and build with WITH_TOOLS and without.

Differential Revision: D20473029

fbshipit-source-id: 3d730fd14bbae6eeeae7f9cc9aec50a4e488ad72

Update HISTORY and version

Use flush time for the props.creation_time for FIFO compaction (#6612)

Summary:
For FIFO compaction, we use flush time instead of oldest key time as the
creation time. This is to prevent FIFO compaction dropping files whose oldest
key time is older than TTL but which has newer keys than TTL.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6612

Test Plan: make check

Reviewed By: siying

Differential Revision: D20748217

Pulled By: riversand963

fbshipit-source-id: 3f7b00a847020760537cdddd12f6fe039e5bc663

Fix data race of GetCreationTimeOfOldestFile() (#6473)

Summary:
When DBImpl::GetCreationTimeOfOldestFile() calls Version::GetCreationTimeOfOldestFile(), the version is not directly or indirectly referenced, so an event like compaction can race with the operation and cause DBImpl::GetCreationTimeOfOldestFile() to access delocated data. This was caught by an ASAN run:

==268==ERROR: AddressSanitizer: heap-use-after-free on address 0x612000b7d198 at pc 0x000018332913 bp 0x7f391510d310 sp 0x7f391510d308
READ of size 8 at 0x612000b7d198 thread T845 (store_load-33)
SCARINESS: 51 (8-byte-read-heap-use-after-free)
    #0 0x18332912 in rocksdb::Version::GetCreationTimeOfOldestFile(unsigned long*) rocksdb/src/db/version_set.cc:1488
    https://github.com/facebook/rocksdb/issues/1 0x1803ddaa in rocksdb::DBImpl::GetCreationTimeOfOldestFile(unsigned long*) rocksdb/src/db/db_impl/db_impl.cc:4499
    https://github.com/facebook/rocksdb/issues/2 0xe24ca09 in rocksdb::StackableDB::GetCreationTimeOfOldestFile(unsigned long*) rocksdb/utilities/stackable_db.h:392
    ......
0x612000b7d198 is located 216 bytes inside of 296-byte region [0x612000b7d0c0,0x612000b7d1e8)
freed by thread T28 here:
    ......
    https://github.com/facebook/rocksdb/issues/5 0x1832c73f in std::vector<rocksdb::FileMetaData*, std::allocator<rocksdb::FileMetaData*> >::~vector() third-party-buck/platform007/build/libgcc/include/c++/trunk/bits/stl_vector.h:435
    https://github.com/facebook/rocksdb/issues/6 0x1832c73f in rocksdb::VersionStorageInfo::~VersionStorageInfo() rocksdb/src/db/version_set.cc:734
    https://github.com/facebook/rocksdb/issues/7 0x1832cf42 in rocksdb::Version::~Version() rocksdb/src/db/version_set.cc:758
    https://github.com/facebook/rocksdb/issues/8 0x9d1bb5 in rocksdb::Version::Unref() rocksdb/src/db/version_set.cc:2869
    https://github.com/facebook/rocksdb/issues/9 0x183e7631 in rocksdb::Compaction::~Compaction() rocksdb/src/db/compaction/compaction.cc:275
    https://github.com/facebook/rocksdb/issues/10 0x9e6de6 in std::default_delete<rocksdb::Compaction>::operator()(rocksdb::Compaction*) const third-party-buck/platform007/build/libgcc/include/c++/trunk/bits/unique_ptr.h:78
    https://github.com/facebook/rocksdb/issues/11 0x9e6de6 in std::unique_ptr<rocksdb::Compaction, std::default_delete<rocksdb::Compaction> >::reset(rocksdb::Compaction*) third-party-buck/platform007/build/libgcc/include/c++/trunk/bits/unique_ptr.h:376
    https://github.com/facebook/rocksdb/issues/12 0x9e6de6 in rocksdb::DBImpl::BackgroundCompaction(bool*, rocksdb::JobContext*, rocksdb::LogBuffer*, rocksdb::DBImpl::PrepickedCompaction*, rocksdb::Env::Priority) rocksdb/src/db/db_impl/db_impl_compaction_flush.cc:2826
    https://github.com/facebook/rocksdb/issues/13 0x9ac3b8 in rocksdb::DBImpl::BackgroundCallCompaction(rocksdb::DBImpl::PrepickedCompaction*, rocksdb::Env::Priority) rocksdb/src/db/db_impl/db_impl_compaction_flush.cc:2320
    https://github.com/facebook/rocksdb/issues/14 0x9abff7 in rocksdb::DBImpl::BGWorkCompaction(void*) rocksdb/src/db/db_impl/db_impl_compaction_flush.cc:2096
    ......

Fix the issue by reference the super version and use the referenced version from it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6473

Test Plan: Run ASAN for all existing tests.

Differential Revision: D20196416

fbshipit-source-id: 5f4a7918110fc7b8dd7841932d376bc9d1e59d6f

Buck config: Re-enable liburing under Linux (#6451)

Summary:
The known bug of liburing has been fixed. Now we can re-enable liburing under Linux
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6451

Test Plan: Watch internal CI

Differential Revision: D20079009

fbshipit-source-id: 04a6f53a900ff721f9a62a188cf906771b5d68d2

Update release version to 6.8 (#6450)

Summary:
Update release version to 6.8
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6450

Test Plan: no code change

Differential Revision: D20071889

Pulled By: cheng-chang

fbshipit-source-id: 91450aae09b201926469ff32f59ed436366f3b74

Share kPageSize (and other small tweaks) (#6443)

Summary:
Make kPageSize extern const size_t (used in draft https://github.com/facebook/rocksdb/issues/6427)
Make kLitteEndian constexpr bool
Clarify a couple of comments
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6443

Test Plan: make check, CI

Differential Revision: D20044558

Pulled By: pdillinger

fbshipit-source-id: e0c5cc13229c82726280dc0ddcba4078346b8418