mon: start lease timer from peon_init()

In the scenario:

 - leader wins, peons lose
 - leader sees it is too far behind on paxos and bootstraps
 - leader tries to sync with someone, waits for a quorum of the others
 - peons sit around forever waiting

The problem is that they never time out because paxos never issues a lease,
which is the normal timeout that lets them detect a leader failure.

Avoid this by starting the lease timeout as soon as we lose the election.
The timeout callback just does a bootstrap and does not rely on any other
state.

I see one possible danger here: there may be some "normal" cases where the
leader takes a long time to issue its first lease that we currently
tolerate, but won't with this new check in place.  I hope that raising
the lease interval/timeout or reducing the allowed paxos drift will make
that a non-issue.  If it is problematic, we will need a separate explicit
"i am alive" from the leader while it is getting ready to issue the lease
to prevent a live-lock.

Backport: cuttlefish, bobtail
Signed-off-by: Sage Weil <sage@inktank.com>