The largest_started_sub_id needs to be set under LOCK_parallel_entry
together with testing stop_sub_id. However, in-between was the logic for
do_ftwrl_wait(), which temporarily releases the mutex. This could lead to
inconsistent stopping amongst worker threads and lost data.

Fix by moving all the stop-related logic out from unrelated do_gco_wait()
and do_ftwrl_wait() and into its own function do_stop_handling().
Reviewed-by: Andrei Elkin <andrei.elkin@mariadb.com>
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>