make compaction_readahead_size_ thread safe
make compaction_readahead_size_ thread safe

Summary:
this should fix the failing tsan_check
Closes https://github.com/facebook/rocksdb/pull/3192

Differential Revision: D6390004

Pulled By: miasantreble

fbshipit-source-id: 6cadfc6f68febb1a77b0abcdb5416570dad926a5

SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB