msg/Pipe: fix seq handshake on reconnect

We go to the trouble to exchange our seq numbers during the handshake, but
the bit that then avoids resending old messages was broken because we
already requeue_sent() before we get to this point.  Fix it by discarding
queued items (in the high prio slot) that we don't need to resend, and
adjust out_seq as needed.

Drop the optional arg to requeue_sent() now that it is unused.

Signed-off-by: Sage Weil <sage@inktank.com>