Summary:
Don't enable ROCKSDB_JEMALLOC unless the build mode is opt and default
allocator is jemalloc. In dev mode, this is causing compile/link errors such as -
```
stderr: buck-out/dev/gen/rocksdb/src/rocksdb_lib#compile-pic-malloc_stats.cc.o4768b59e,gcc-5-glibc-2.23-clang/db/malloc_stats.cc.o:malloc_stats.cc:function rocksdb::DumpMallocStats(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*): error: undefined reference to 'malloc_stats_print'
clang-7.0: error: linker command failed with exit code 1
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4747

Differential Revision: D13324840

Pulled By: anand1976

fbshipit-source-id: 45ffbd4f63fe4d9e8a0473d8f066155e4ef64a14