break Flush wait for dropped CF

Summary:
In FlushJob we dont do the Flush if the CF is dropped
https://github.com/facebook/rocksdb/blob/master/db/flush_job.cc#L184-L188

but inside WaitForFlushMemTable we keep waiting forever even if the CF is dropped.
Closes https://github.com/facebook/rocksdb/pull/1664

Differential Revision: D4321032

Pulled By: IslamAbdelRahman

fbshipit-source-id: 6e2b25d

db/db_impl.cc

@@ -2937,6 +2937,12 @@ Status DBImpl::WaitForFlushMemTable(ColumnFamilyData* cfd) {
     if (shutting_down_.load(std::memory_order_acquire)) {
       return Status::ShutdownInProgress();
     }
+    if (cfd->IsDropped()) {
+      // FlushJob cannot flush a dropped CF, if we did not break here
+      // we will loop forever since cfd->imm()->NumNotFlushed() will never
+      // drop to zero
+      return Status::InvalidArgument("Cannot flush a dropped CF");
+    }
     bg_cv_.Wait();
   }
   if (!bg_error_.ok()) {