Make a copy of MutableCFOptions to avoid race condition (#4876)

Make a copy of MutableCFOptions to avoid race condition (#4876)

Summary:
If we do not do this, then reading MutableCFOptions may have a race condition
with SetOptions which modifies MutableCFOptions.

Also reserve space in advance for vectors to avoid reallocation changing the
address of its elements.

Test plan
```
$make clean && make -j32 all check
$make clean && COMPILE_WITH_TSAN=1 make -j32 all check
$make clean && COMPILE_WITH_ASAN=1 make -j32 all check
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4876

Differential Revision: D13644500

Pulled By: riversand963

fbshipit-source-id: 4b8112c5c819d5a2922bb61ad1521b3d2fb2fd47