local_global_read_lock did release all HANDLER's before taking its
MDL_BACKUP_FTWRL# locks. This had a potential race condition if
there was a waiting LOCK TABLE for one of the freed handlers.

Fixed by moving the release of HANDLER's to after the backup locks are
taken.

After commit 8cf7e345 it's not anymore critical to free HANDLER's in
FTWRL, but we will keep the mysql_ha_cleanup_no_free() call until 10.5
to not cause any issues with 10.4 just before it's going GA.