mgr/vol: add pause/resume mechanism for async jobs

mgr/vol: add pause/resume mechanism for async jobs

Add mechansim that allows pausing/resuming of the entire async job
machinery that queues, launches and picks next async job; both async
jobs, clones as well as purges.

And then add mgr/vol config option pause_purging and pause_cloning so
that both of these async jobs can be paused and resumed individually.

Fixes: https://tracker.ceph.com/issues/61903
Fixes: https://tracker.ceph.com/issues/68630
Signed-off-by: Rishabh Dave <ridave@redhat.com>