Summary:
In the current trace replay, all the queries are serialized and called by single threads. It may not simulate the original application query situations closely. The multi-threads replay is implemented in this PR. Users can set the number of threads to replay the trace. The queries generated according to the trace records are scheduled in the thread pool job queue.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5934

Test Plan: test with make check and real trace replay.

Differential Revision: D17998098

Pulled By: zhichao-cao

fbshipit-source-id: 87eecf6f7c17a9dc9d7ab29dd2af74f6f60212c8