mon: fix pg stats timeout

mon: fix pg stats timeout

We clear out the osd entry when an osd goes up or down.  Thus, if we find
it missing from an up osd, we should start the timer.  Otherwise we get
behavior like this

2012-04-24 13:22:47.888291 7fa5bc587700 mon.peon5752@0(leader).osd e21633 OSDMonitor::handle_osd_timeouts: never got MOSDPGStat info from osd 521. Marking down!
2012-04-24 13:22:50.076394 7fa5bcd88700 log [INF] : osd.521 [2607:f298:4:2243::7088]:6806/53217 boot
2012-04-24 13:22:52.903558 7fa5bc587700 mon.peon5752@0(leader).osd e21638 OSDMonitor::handle_osd_timeouts: never got MOSDPGStat info from osd 521. Marking down!
2012-04-24 13:23:15.144532 7fa5bcd88700 log [INF] : osd.521 [2607:f298:4:2243::7088]:6806/53217 boot
2012-04-24 13:23:17.967118 7fa5bc587700 mon.peon5752@0(leader).osd e21663 OSDMonitor::handle_osd_timeouts: never got MOSDPGStat info from osd 521. Marking down!
2012-04-24 13:23:22.173778 7fa5bcd88700 log [INF] : osd.521 [2607:f298:4:2243::7088]:6806/53217 boot
2012-04-24 13:23:22.981556 7fa5bc587700 mon.peon5752@0(leader).osd e21668 OSDMonitor::handle_osd_timeouts: never got MOSDPGStat info from osd 521. Marking down!
2012-04-24 13:23:45.245380 7fa5bcd88700 log [INF] : osd.521 [2607:f298:4:2243::7088]:6806/53217 boot

when the pg stats message doesn't arrive quickly enough.

Fixes: #2341
Signed-off-by: Sage Weil <sage@newdream.net>
Reviewed-by: Greg Farnum <gregory.farnum@dreamhost.com>