Objecter: respect higher epoch subscription in tick

Objecter: respect higher epoch subscription in tick

The OSD and Objecter share the same MonClient. During preboot, a potential
race condition exists where the OSD subscribes to osdmap epoch X, while
the Objecter subscribes to epoch X - 1.

The Objecter's subscription overrides the OSD's subscription. Consequently,
the monitor ignores the request (as it believes the OSD already has the
older map), causing the OSD to hang during preboot.

To fix this, check if a higher epoch is already subscribed before calling
_maybe_request_map during Objecter::tick. If a higher epoch is found,
maintain the existing subscription.

Fixes: https://tracker.ceph.com/issues/71931
Signed-off-by: Nitzan Mordechai <nmordech@ibm.com>