Changing behaviour so that instead of rebinding after a successful
migration and continuing as an ordinary server, we simply quit with a
0 exit code and let our caller restart us as a server if they want to.
This means that everything in listen.c, listen.h, and anything making
reference to a rebind address is unneeded.
This adds a test for destination behaviour, in that if a source crashes
after sending an entrust message but before the destination can reply,
the destination must allow the source to reconnect and retry the mirror.
