osd: wait for healthy pings from peers in waiting-for-healthy state

osd: wait for healthy pings from peers in waiting-for-healthy state

If we are (wrongly) marked down, we need to go into the waiting-for-healthy
state and verify that our network interfaces are working before trying to
rejoin the cluster.

 - make _is_healthy() check require positive proof of pings working
 - do heartbeat checks and updates in this state
 - reset the random peers every heartbeat_interval, in case we keep picking
   bad ones

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