tools/cephfs_mirror: update checkpoint status during snapshot sync

When the sync completes or fails for a checkpointed snapshot, transition
the status of that checkpoint from CREATED/FAILED to COMPLETE/FAILED in
the do_sync_snaps() along with the timestamp of the event.

Fixes: https://tracker.ceph.com/issues/73454
Signed-off-by: Karthik U S <karthik.u.s1@ibm.com>