osd: augment osd heartbeat peers with neighbors and randoms, to up some min

- always include our neighbors to ensure we have a fully-connected
  graph
- include some random neighbors to get at least some min number of peers.

Signed-off-by: Sage Weil <sage@inktank.com>
osd: factor out _remove_heartbeat_peer

Signed-off-by: Sage Weil <sage@inktank.com>
osd: only add pg peers if active

We will soon be in this method for the waiting-for-healthy state.  As
a consequence, we need to remove any down peers.

Signed-off-by: Sage Weil <sage@inktank.com>
osd: remove down hb peers

If a (say, random) peer goes down, filter it out.

Signed-off-by: Sage Weil <sage@inktank.com>
osd: distinguish between definitely healthy and definitely not unhealthy

is_unhealthy() will assume they are healthy for some period after we
send our first ping attempt.  is_healthy() is now a strict check that we
know they are healthy.

Switch the failure report check to use is_unhealthy(); use is_healthy()
everywhere else, including the waiting-for-healthy pre-boot checks.

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