Fixing occasional MyRocks SIGABRT on shutdown (mysql#122)

Summary:
RocksDB has some static object dependencies, and without
following dependencies, it hits SIGABRT. This diff has
two fixes.
1. Updating RocksDB revision. D51789 in RocksDB changes the problematic
mutex from global variable to local (function-level) static variable.
2. Changing MyRocks no to call ddl_manager.persist_stats() by
kill_server_thread (which calls rocksdb_done_func()). By calling
ddl_manager.persist_stats(),
rocksdb::ThreadLocalPtr::StaticMeta::OnThreadExit() (which calls
pthread_mutex_lock on "static port::Mutex mutex") is called at the end
of the thread. On the other hand, there is a race between main thread and
kill_server_thread. main thread calls pthread_mutex_destroy for the
mutex. To guarantee not to call pthread_mutex_lock() after
pthread_mutex_destroy(), it is necessary not to call
ddl_manager.persist_stats() from kill_server_thread. This diff fixes the
issue.

@update-submodule: rocksdb

Test Plan: mtr cardinality --repeat=100

Reviewers: hermanlee4, spetrunia, jkedgar

Reviewed By: jkedgar

Subscribers: yhchiang, webscalesql-eng

Differential Revision: https://reviews.facebook.net/D51933

Author: yoshinorim

storage/rocksdb/ha_rocksdb.cc

@@ -2402,9 +2402,6 @@ static int rocksdb_done_func(void *p)
   mysql_mutex_lock(&background_mutex);
   mysql_mutex_unlock(&background_mutex);
 
-  // save remaining stats which might've left unsaved
-  ddl_manager.persist_stats();
-
   // wait for the drop index thread to finish
   mysql_mutex_lock(&drop_index_mutex);
   mysql_mutex_unlock(&drop_index_mutex);
@@ -6987,6 +6984,9 @@ void* background_thread(void*)
     }
   }
 
+  // save remaining stats which might've left unsaved
+  ddl_manager.persist_stats();
+
   mysql_mutex_unlock(&background_mutex);
 
   return nullptr;