crimson/osd: keep track of last_peering_reset

crimson/osd: keep track of last_peering_reset

last_peering_reset is used to track the epoch when the most recent peering
started. it is used to tell if a message is targeting a changed PG. the
sender of a peering message will attach it. and the recipient will check
it against its own last_peering_reset. for instance, if a message claims
that that its map epoch is 16, while our last_peering_reset is 18, we will
consider this message stale, and probably drop it.

Signed-off-by: Kefu Chai <kchai@redhat.com>