mon: mark osds down for not sending MOSDPGStat

mon: mark osds down for not sending MOSDPGStat

PGMonitor::prepare_pg_stats should check to see if the stats in the
MOSDPgStats message are the same as the ones we already have. If so, no
need to create an incremental; just send an ACK and return false.

The leading Monitor now marks osds as down if they haven't sent a
MOSDPGStat message in the last 15 minutes.

Signed-off-by: Colin McCabe <colinm@hq.newdream.net>