mon: drop forwarded requests after an election

mon: drop forwarded requests after an election

On each election, we resend routed requests to the new leader (or
requeue for ourselves).  Therefore, if we receive a forwarded request,
we should drop it on the floor if there is a new election.  Add a field
in the PaxosServiceMessage struct to track which election epoch we
received the request in, and drop it in PaxosService::dispatch() if
that is in the past.

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