mon: assert that we are not double-forwarding

We should never forward a request that has already been forwarded.  In
theory we could by setting proxy_con here, but we don't want to.  In
general, we resend forwarded requests on election boundaries and do not
resend.

There is one case where we forward MLog from ourself/loopback to the leader
but that is not a double-forward either; in that case the rr->con here is
the loopback con.

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