mon/Elector: force election epoch bump on start

mon/Elector: force election epoch bump on start

We are generally careful when bumping the epoch so that we can join
existing rounds.  However, if we restart in the middle of an election,
and change versions, we need to be certain that our previous ACK (as
$version - 1) isn't accepted as truth for the restarted daemon (running
$version) keeping the same epoch.

The conservatism with bumping is to avoid spurious election cycles, but
mon restarts are more rare, and we need them here.

Fixes: http://tracker.ceph.com/issues/20949
Signed-off-by: Sage Weil <sage@redhat.com>