rocksdb:Fixes for http://tracker.ceph.com/issues/20529

bump version to 5.6

Summary:
Bump version to 5.6 beforehand in master
Closes https://github.com/facebook/rocksdb/pull/2411

Differential Revision: D5186896

Pulled By: lightmark

fbshipit-source-id: 079538e621b1a959c2dc99dada894e9cdb99ef95

Force travis to build with clang on MacOS

Summary:
Attempt to force travis to build with clang on MacOS
Closes https://github.com/facebook/rocksdb/pull/2408

Differential Revision: D5186635

Pulled By: yiwu-arbug

fbshipit-source-id: dbb779eff07b1cb7dbd2092631303cf946316656

Fix warnings while generating RocksJava documentation

Summary:
There are a couple of warnings while building RocksJava, coming from Javadoc generation.
```
Generating target/apidocs/org/rocksdb/RocksDB.html...
src/main/java/org/rocksdb/RocksDB.java:2139: warning: no throws for org.rocksdb.RocksDBException
  public void ingestExternalFile(final List<String> filePathList,
              ^
src/main/java/org/rocksdb/RocksDB.java:2162: warning: no throws for org.rocksdb.RocksDBException
  public void ingestExternalFile(final ColumnFamilyHandle columnFamilyHandle,
              ^
```
Closes https://github.com/facebook/rocksdb/pull/2396

Differential Revision: D5178388

Pulled By: sagar0

fbshipit-source-id: a0ab6696d6de78d089a9a860a559f64cc320019e

WriteOptions.low_pri which can throttle low pri writes if needed

Summary:
If ReadOptions.low_pri=true and compaction is behind, the write will either return immediate or be slowed down based on ReadOptions.no_slowdown.
Closes https://github.com/facebook/rocksdb/pull/2369

Differential Revision: D5127619

Pulled By: siying

fbshipit-source-id: d30e1cff515890af0eff32dfb869d2e4c9545eb0

Switch from CentOS 5 to CentOS 6 for crossbuilding RocksJava

Summary:
Updates the statically linked libraries from linking against glibc 2.5, to linking against glibc 2.12.
Closes https://github.com/facebook/rocksdb/pull/2405

Differential Revision: D5184132

Pulled By: sagar0

fbshipit-source-id: 7a8ad4cf7e737ca62f29e58938bd49fa02114541

Fix db_write_test clang/windows build failure

Summary:
Fix db_write_test clang/windows build failure. Explicitly cast size_t (unsigned long) to uint32_t (unsigned int).
Closes https://github.com/facebook/rocksdb/pull/2407

Differential Revision: D5182995

Pulled By: yiwu-arbug

fbshipit-source-id: aba225a9fccb12d5bfbdc2cd6efc11040706a9d2

fixed typo

Summary:
fixed typo
Closes https://github.com/facebook/rocksdb/pull/2376

Differential Revision: D5183630

Pulled By: ajkr

fbshipit-source-id: 133cfd0445959e70aa2cd1a12151bf3c0c5c3ac5

Fix the Java build which was broken by a4d9c02

Summary: Closes https://github.com/facebook/rocksdb/pull/2406

Differential Revision: D5181091

Pulled By: ajkr

fbshipit-source-id: fd72525da4fb1d50143080a210f8d824cbb968d6

remove test dir before exit when current regression is running

Summary:
clean up the current test dir if the last regression test is still running.
Closes https://github.com/facebook/rocksdb/pull/2401

Differential Revision: D5177882

Pulled By: lightmark

fbshipit-source-id: 91d899fcc2bde841948eae71af8584d4bdb35468