Add lock argument to server_service::background_rollback()

This is to allow implementation to temporarily release the
lock if needed.

src/transaction.cpp

@@ -1073,7 +1073,7 @@ bool wsrep::transaction::bf_abort(
                 }
             }
 
-            server_service_.background_rollback(client_state_);
+            server_service_.background_rollback(lock, client_state_);
         }
     }
     return ret;