mon: fix monitor cluster contraction race
If we contract to 1 monitor, we win_standalone_election() without bumping
the election epoch. Racing paxos updates can then reach us without being
ignored and trigger an assert:
mon/Paxos.cc: In function 'void Paxos::handle_accept(MMonPaxos*)' thread
7f85eae05700 time 2012-08-20 16:01:00.843937
mon/Paxos.cc: 468: FAILED assert(state == STATE_UPDATING)
Fixes: #3003
Reported-by: John Wilkins <john.wilkins@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>