msg/Pipe: fix RECONNECT_SEQ behavior

Calling handle_ack() here has no effect because we have already
spliced sent messages back into our out queue.  Instead, pull them out
of there and discard.  Add a few assertions along the way.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>