Avoid too frequent MaybeScheduleFlushOrCompaction() call

Summary:
If there's manual compaction in the queue, then "HaveManualCompaction(compaction_queue_.front())" will return true, and this cause too frequent MaybeScheduleFlushOrCompaction().

https://github.com/facebook/rocksdb/issues/3198
Closes https://github.com/facebook/rocksdb/pull/3366

Differential Revision: D6729575

Pulled By: ajkr

fbshipit-source-id: 96da04f8fd33297b1ccaec3badd9090403da29b0

db/db_impl_compaction_flush.cc

@@ -1506,7 +1506,7 @@ Status DBImpl::BackgroundCompaction(bool* made_progress,
                : m->manual_end->DebugString().c_str()));
     }
   } else if (!is_prepicked && !compaction_queue_.empty()) {
-    if (HaveManualCompaction(compaction_queue_.front())) {
+    if (HasExclusiveManualCompaction()) {
       // Can't compact right now, but try again later
       TEST_SYNC_POINT("DBImpl::BackgroundCompaction()::Conflict");