DB::Flush() Do not wait for background threads when there is nothing in mem table

Summary:
When we have multiple column families, users can issue Flush() on every column families to make sure everything is flushes, even if some of them might be empty. By skipping the waiting for empty cases, it can be greatly speed up.

Still wait for people's comments before writing unit tests for it.

Test Plan: Will write a unit test to make sure it is correct.

Reviewers: ljin, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D22953