AKA rpl.rpl_parallel, binlog_encryption.rpl_parallel fails in
buildbot with timeout in include

A replication parallel worker thread can deadlock with another
connection running SHOW SLAVE STATUS. That is, if the replication
worker thread is in do_gco_wait() and is killed, it will already
hold the LOCK_parallel_entry, and during error reporting, try to
grab the err_lock. SHOW SLAVE STATUS, however, grabs these locks in
reverse order. It will initially grab the err_lock, and then try to
grab LOCK_parallel_entry. This leads to a deadlock when both threads
have grabbed their first lock without the second.

This patch implements the MDEV-31894 proposed fix to optimize the
workers_idle() check to compare the last in-use relay log’s
queued_count==dequeued_count for idleness. This removes the need for
workers_idle() to grab LOCK_parallel_entry, as these values are
atomically updated.

Huge thanks to Kristian Nielsen for diagnosing the problem!

Reviewed By:
============
Kristian Nielsen <knielsen@knielsen-hq.org>
Andrei Elkin <andrei.elkin@mariadb.com>