Fix data loss after DB recovery by not allowing flush/compaction to be scheduled...

Fix data loss after DB recovery by not allowing flush/compaction to be scheduled until DB opened

Summary:
Previous run may leave some SST files with higher file numbers than manifest indicates.
Compaction or flush may start to run while DB::Open() is still going on. SST file garbage collection may happen interleaving with compaction or flush, and overwrite files generated by compaction of flushes after they are generated. This might cause data loss. This possibility of interleaving is recently introduced.
Fix it by not allowing compaction or flush to be scheduled before DB::Open() finishes.

Test Plan: Add a unit test. This verification will have a chance to fail without the fix but doesn't fix without the fix.

Reviewers: kradhakrishnan, anthony, yhchiang, IslamAbdelRahman, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D42399