Run compactions even if workload is readonly or read-mostly.

Summary:
The events that trigger compaction:
* opening the database
* Get -> only if seek compaction is not disabled and other checks are true
* MakeRoomForWrite -> when memtable is full
* BackgroundCall ->
  If the background thread is about to do a compaction run, it schedules
  a new background task to trigger a possible compaction. This will cause
  additional background threads to find and process other compactions that
  can run concurrently.

Test Plan: ran db_bench with overwrite and readonly alternatively.

Reviewers: sheki, MarkCallaghan

Reviewed By: sheki

CC: leveldb

Differential Revision: https://reviews.facebook.net/D9579

db/db_impl.cc

@@ -1294,6 +1294,7 @@ Status DBImpl::BackgroundCompaction(bool* madeProgress,
     versions_->ReleaseCompactionFiles(c.get(), status);
     *madeProgress = true;
   } else {
+    MaybeScheduleCompaction(); // do more compaction work in parallel.
     CompactionState* compact = new CompactionState(c.get());
     status = DoCompactionWork(compact);
     CleanupCompaction(compact);