mgr: relax "pending_service_map.epoch > service_map.epoch" assert

mgr: relax "pending_service_map.epoch > service_map.epoch" assert

When we are activating we may receive several service map updates
initiated by the previous active mgr. Treat them all as initial map.

The code also adds "pending_service_map_dirty == 0" assert, which we
expect is true when receiving an initial map -- otherwise we can't
just initialize pending_service_map with received map.

Fixes: https://tracker.ceph.com/issues/51835
Signed-off-by: Mykola Golub <mgolub@suse.com>
(cherry picked from commit cc2721ccdb33248a732abd1919df808ef8a1f80f)