mgr/rbd_support: Stagger mirror snapshot and trash purge schedules

mgr/rbd_support: Stagger mirror snapshot and trash purge schedules

Previously, multiple images or namespaces scheduled with the same
interval ran mirror snapshots or trash purges at around the same time,
creating spikes in cluster activity.

This change staggers scheduled jobs by:

- Adding a deterministic phase offset per image or namespace when no
  start-time is set.
- Picking a random element from the queue at each scheduled time, rather
  than always the first.

Together, these changes spread snapshot and trash purge operations more
evenly over time and improve cluster stability.

Fixes: https://tracker.ceph.com/issues/74288
Signed-off-by: Ramana Raja <rraja@redhat.com>