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.

(Although it isn't possible to create an identity yet...)

The invitation and private messaging UIs are currently broken. Some key
rotation bugs were fixed; others may have been created (unit tests
needed). An encoding for private keys was added. Pseudonyms were moved
out of the messaging package and ratings were moved in.

The Tor plugin tests are consistently failing with the latest versions
of Silvertunnel and Spongy Castle; we may need to ship native Tor
binaries for Windows/Mac/Linux and use Orbot on Android.