mesa: migrate and handle halted flows

[yosoyubik]

• if a flow had %halt = %.y, migrating the peer to %mesa would ignore it
• for re-sends of %mesa $pleas that had already pending-vane-ack as %.y we would not attempt to %pass the synthetic /gp $plea
• in %ames, if the last fragment of a $plea was re-send, we were looking at live-messages (which is empty for re-sends) instead of pending-vane-ack to try to retrieve the fully assembled message, and pass the synthetic /gp $plea with the offending agent