mon: slurp latest state from active monitors before joining quorum

mon: slurp latest state from active monitors before joining quorum

If a monitor has been down and is behind, and joins the quorum, the
other nodes will try to send it all of the needed state, which can
bring the cluster to a halt.

Instead, implement a new bootstrap() procedure:

 - probe the cluster nodes
 - if there is an existing quorum,
   - and it is not too far ahead of me, join it (call an election)
   - otherwise, slurp down all the newer state and then restart (bootstrap)
 - if we see enough online nodes that are not part of the quorum, call
   an election.

We still need to add some timeouts.

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