Summary: Add a utility function that trigger necessary full compaction and put output to the correct level by looking at new options and old options.

Test Plan: Add unit tests for it.

Reviewers: andrewkr, igor, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: muthu, sumeet, leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D60783