Do not release and re-acquire dbmutex on memtable-switch if no listener (#9758)

Summary:
There is no need to release-and-acquire immediately when no listener is registered. This is
what we have been doing for `NotifyOnFlushBegin()`, `NotifyOnFlushCompleted()`, `NotifyOnCompactionBegin()`,
`NotifyOnCompactionCompleted()`, and some other `NotifyOnXX` methods in event_helpers.cc.
Do the same for `NotifyOnMemTableSealed ()`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9758

Test Plan: make check

Reviewed By: jay-zhuang

Differential Revision: D35159552

Pulled By: riversand963

fbshipit-source-id: 6e0aac50bd5c8f506d809b6638c33a7a28d1e87f

db/db_impl/db_impl_write.cc

@@ -1882,9 +1882,11 @@ void DBImpl::NotifyOnMemTableSealed(ColumnFamilyData* /*cfd*/,
     return;
   }
 
+  mutex_.Unlock();
   for (auto listener : immutable_db_options_.listeners) {
     listener->OnMemTableSealed(mem_table_info);
   }
+  mutex_.Lock();
 }
 #endif  // ROCKSDB_LITE
 
@@ -2085,11 +2087,9 @@ Status DBImpl::SwitchMemtable(ColumnFamilyData* cfd, WriteContext* context) {
                                      mutable_cf_options);
 
 #ifndef ROCKSDB_LITE
-  mutex_.Unlock();
   // Notify client that memtable is sealed, now that we have successfully
   // installed a new memtable
   NotifyOnMemTableSealed(cfd, memtable_info);
-  mutex_.Lock();
 #endif  // ROCKSDB_LITE
   // It is possible that we got here without checking the value of i_os, but
   // that is okay.  If we did, it most likely means that s was already an error.