DB::Open() to automatically increase thread pool size if it is smaller than max number of parallel compactions or flushes

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