After two concurrent FTWRL/UNLOCK TABLES, the node stays in paused state
and the following CREATE TABLE fails with

  ER_UNKNOWN_COM_ERROR (1047): Aborting TOI: Replication paused on
  node for FTWRL/BACKUP STAGE.

The cause is the use of global `wsrep_locked_seqno` to determine
if the node should be resumed on UNLOCK TABLES. In some executions
the `wsrep_locked_seqno` is cleared by the first UNLOCK TABLES
after the second FTWRL gets past `make_global_read_lock_block_commit()`.

As a fix, use `thd->wsrep_desynced_backup_stage` to determine
if the thread should resume the node on UNLOCK TABLES.

Add MTR test galera.galera_ftwrl_concurrent to reproduce the
race. The test contains also cases for BACKUP STAGE which
uses similar mechanism for desyncing and pausing the node.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>