fix hanging after CompactFiles with L0 overlap

fix hanging after CompactFiles with L0 overlap

Summary:
Bug report: https://www.facebook.com/groups/rocksdb.dev/permalink/1389452781153232/

Non-empty `level0_compactions_in_progress_` was aborting `CompactFiles` after incrementing `bg_compaction_scheduled_`, and in that case we never decremented it. This blocked future compactions and prevented DB close as we wait for scheduled compactions to finish/abort during close.

I eliminated `CompactFiles`'s dependency on `level0_compactions_in_progress_`. Since it takes a contiguous span of L0 files -- through the last L0 file if any L1+ files are included -- it's fine to run in parallel with other compactions involving L0. We make the same assumption in intra-L0 compaction.
Closes https://github.com/facebook/rocksdb/pull/2849

Differential Revision: D5780440

Pulled By: ajkr

fbshipit-source-id: 15b15d3faf5a699aed4b82a58352d4a7bb23e027