Only attempt to flush queue if the underlying worker pool is not finished (#18593...

Only attempt to flush queue if the underlying worker pool is not finished (#18593) (#18620)

* Only attempt to flush queue if the underlying worker pool is not finished (#18593)

Backport #18593

There is a possible race whereby a worker pool could be cancelled but yet the
underlying queue is not empty. This will lead to flush-all cycling because it
cannot empty the pool.

* On shutdown of Persistant Channel Queues close datachan and empty

Partial Backport #18415

Although we attempt to empty the datachan in queues - due to
races we are better off just closing the channel and forcibly emptying
it in shutdown.

Fix #18618

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Move zero workers warning to debug

Fix #18617

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Update modules/queue/manager.go

Co-authored-by: Gusted <williamzijl7@hotmail.com>
* Update modules/queue/manager.go

Co-authored-by: Gusted <williamzijl7@hotmail.com>
Co-authored-by: Gusted <williamzijl7@hotmail.com>