Summary:
Since level 0 files can overlap, two level 0 compactions cannot
run in parallel. Compact files needs to check this before running a
compaction.

Test Plan: CompactFilesTest.L0ConflictsFiles

Reviewers: igor, IslamAbdelRahman, anthony, sdong, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D50079