Summary: CompactRange() now is much more expensive for dynamic level base size as it goes through all the levels. Skip those not used levels between level 0 an base level.

Test Plan: Run all unit tests

Reviewers: yhchiang, rven, anthony, kradhakrishnan, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D37125