librbd: update hidden global config when setting pool config override

librbd: update hidden global config when setting pool config override

The new "dev"-level global config setting will be updated when any
pool-level config override is updated. librbd clients will detect
the new global-level config update and trigger a refresh. This avoids
the need for potentially tens of thousands of librbd clients
registering a watch on the pool metadata object or periodically polling
the pool metadata object for updates.

Fixes: https://tracker.ceph.com/issues/46694
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit f45df9fe786e8057c491c082e840483759d67e9e)

Conflicts:
	src/common/options.cc
- "rbd_quiesce_notification_attempts", "rbd_default_snapshot_quiesce_mode", and
  "rbd_plugins" options have not been backported to Octopus, yet