Add notion of `ephemeral' associations and a goodbye protocol.

When TrIPE kills an ephemeral peer, it sends a `bye' message to the
peer.  When TrIPE receives `bye' from an ephemeral peer, it kills the
peer (without sending `bye' back).

Augment the `connect' service to set appropriate flags when adding
peers, and the Wireshark dissector to understand the new message.