Fix DBImpl::NotifyOnCompactionCompleted data race
Fix DBImpl::NotifyOnCompactionCompleted data race

Summary:
Access of `cfd->current()` needs to hold db mutex. The data race is caught by TSAN but hard to reproduce: https://gist.github.com/yiwu-arbug/0fc6dc0de915297a1740aa9610be9373
Closes https://github.com/facebook/rocksdb/pull/2894

Differential Revision: D5843884

Pulled By: yiwu-arbug

fbshipit-source-id: 0a30a421bc96f51840821538ad6453dc0815a942

bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb