MDEV-15372: Parallel slave speedup very limited when log_slave_updates=OFF

Make MyRocks' non-XA commit path to first do the commit without syncing and then sync.

Author: spetrunia

storage/rocksdb/ha_rocksdb.cc

@@ -3154,10 +3154,20 @@ static int rocksdb_commit(handlerton* hton, THD* thd, bool commit_tx)
  - For a COMMIT statement that finishes a multi-statement transaction
  - For a statement that has its own transaction
  */
+
+ // First, commit without syncing. This establishes the commit order
+ tx->set_sync(false);
  if (tx->commit()) {
  DBUG_RETURN(HA_ERR_ROCKSDB_COMMIT_FAILED);
  }
  thd_wakeup_subsequent_commits(thd, 0);
+
+ if (rocksdb_flush_log_at_trx_commit == FLUSH_LOG_SYNC)
+ {
+ rocksdb::Status s= rdb->FlushWAL(true);
+ if (!s.ok())
+ DBUG_RETURN(HA_ERR_INTERNAL_ERROR);
+ }
  } else {
  /*
  We get here when committing a statement within a transaction.