Find the newest dead secret for each endpoint, erase any others and
derive from the newest.

This bug was causing crashes at shutdown when the connection recogniser
tried to derive tags from secrets that had been erased by the key
manager - the derived tags were not present in the recogniser's maps.

Returning null from getItem() was causing too many problems, and the
item was triggering onItemSelected() before the other items had been
loaded.

Some packet types weren't being generated or handled, connections
weren't properly disposed of when exceptions occurred.

Some devices don't support IPv6, so we'll need to bind separate IPv4 and
IPv6 sockets and advertise both addresses.