MDEV-21329 InnoDB: Failing assertion: lock->lock_word.load(std::memory_order_relaxed) == X_LOCK_DECR upon server shutdown

Problem is that dropping of fts table and sync of fts table
happens concurrently during fts optimize thread shutdown.
fts_optimize_remove_table() is executed by a user thread that
performs DDL, and that the fts_optimize_wq may be removed if
fts_optimize_shutdown() has started executing.

fts_optimize_remove_table() doesn't remove the table from the queue
and it leads to above scenario. While removing the table from
fts_optimize_wq, if the table can't be removed then wait till
fts_optimize_thread shuts down.

Reviewed-by: Marko Mäkelä