MDEV-34493 memory leak when using galera node with only replication thread

Applying of write set containing many row events causes
applier `thd->mem_root` to grow until applying is over.
This may contribute to memory fragmentation problem, producing
large chunks of memory which are not released back to operating
system.

In order to reduce memory fragmentation, free `thd->mem_root`
in `Rows_log_event::do_apply_event()` after applying a row event
with `F_STMT_END` set.

Author: temeo

sql/log_event_server.cc

@@ -5969,7 +5969,11 @@ int Rows_log_event::do_apply_event(rpl_group_info *rgi)
       slave_rows_error_report(ERROR_LEVEL, thd->is_error() ? 0 : error,
                               rgi, thd, table, get_type_str(),
                               RPL_LOG_NAME, log_pos);
-    if (thd->slave_thread)
+    if (thd->slave_thread
+#ifdef WITH_WSREP
+        || (WSREP(thd) && wsrep_thd_is_applying(thd))
+#endif /* WITH_WSREP */
+    )
       free_root(thd->mem_root, MYF(MY_KEEP_PREALLOC));
   }