mon: change timecheck_* key to int

mon: change timecheck_* key to int

We don't need to use entity_inst_t here since we're only paying attention
to mons in the quorum.

This involves changing the message format.  To save some time/effort and
complexity, we only support time checks on all-nautilus.  If you are
mid-upgrade, the time sync checks are skipped.

Signed-off-by: Sage Weil <sage@redhat.com>