The auth_reply handler will (re)send any pending requests.  For the
initial mon authenticate phase, that's correct, but when a auth ticket
renewal races with an in-flight request, we may resend a request message
that is already in flight.  Avoid this by revoking the message before
sending it.

We should also avoid resending requests at all during ticket renewal; that
will come soon.
Signed-off-by: Sage Weil <sage@newdream.net>