Fix 2PC with concurrent memtable insert

Summary:
If concurrent memtable insert is enabled, and one prepare command and a normal command are grouped into a commit group, the sequence ID will be calculated incorrectly.
Closes https://github.com/facebook/rocksdb/pull/1730

Differential Revision: D4371081

Pulled By: siying

fbshipit-source-id: cd40c6d

db/write_thread.cc

@@ -317,7 +317,7 @@ void WriteThread::LaunchParallelFollowers(ParallelGroup* pg,
 
   while (w != pg->last_writer) {
     // Writers that won't write don't get sequence allotment
-    if (!w->CallbackFailed()) {
+    if (!w->CallbackFailed() && w->ShouldWriteToMemtable()) {
       sequence += WriteBatchInternal::Count(w->batch);
     }
     w = w->link_newer;