Summary:
Issue scenario:
(1) We have 3 files in L1 and we issue a compaction that will compact them into 1 file in L2
(2) While compaction (1) is running, we flush a file into L0 and trigger another compaction that decide to move this file to L1 and then move it again to L2 (this file don't overlap with any other files)
(3) compaction (1) finishes and install the file it generated in L2, but this file overlap with the file we generated in (2) so we break the LSM consistency

Looks like this issue can be triggered by using non-exclusive manual compaction or AddFile()

Test Plan: unit tests

Reviewers: sdong

Reviewed By: sdong

Subscribers: hermanlee4, jkedgar, andrewkr, dhruba, yoshinorim

Differential Revision: https://reviews.facebook.net/D64947