MDEV-18080, part#1: MyRocks is slow with log-bin=off

The cause for this was fix MDEV-15372, which was trying to speed up
the parallel slave.

Part#1: Do not attempt the "optimization" for transactions that are not
replication slave workers.

storage/rocksdb/ha_rocksdb.cc

@@ -3750,6 +3750,10 @@ 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
       */
+      if (thd->slave_thread)
+      {
+        // An attempt to make parallel slave performant (not fully successful,
+        // see MDEV-15372):
 
         //  First, commit without syncing. This establishes the commit order
         tx->set_sync(false);
@@ -3765,6 +3769,14 @@ static int rocksdb_commit(handlerton* hton, THD* thd, bool commit_tx)
           if (!s.ok())
             DBUG_RETURN(HA_ERR_INTERNAL_ERROR);
         }
+      }
+      else
+      {
+        /* Not a slave thread */
+        if (tx->commit()) {
+          DBUG_RETURN(HA_ERR_ROCKSDB_COMMIT_FAILED);
+        }
+      }
     } else {
       /*
         We get here when committing a statement within a transaction.