osd: make sure hb peers get marked down

osd: make sure hb peers get marked down

We mark_down on osdmap update when we see an osd has gone down, but the
heartbeats are sent in a different thread without map_lock using
heartbeat_inst.  So, make sure heartbeat_inst entries are removed.

Also, we add hb peers at peers' request.  When removing such entries in
update_heartbeat_peers, mark_down then, too.  (We may mark_down a failed
peer, and then receive the hb request late.  So we mark that down next
time we update the heartbeat maps.)