Summary:
With the patch, thread pool size will be automatically increased if DB's options ask for more parallelism of compactions or flushes.
Too many users have been confused by the API. Change it to make it harder for users to make mistakes
Test Plan: Add two unit tests to cover the function.
Reviewers: yhchiang, rven, igor, MarkCallaghan, ljin
Reviewed By: ljin
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D27555