This makes it more likely that connections will be closed cleanly.
However, the interrupt() method is currently ineffective for incoming
sessions as it won't interrupt a blocking read, e.g. when the packet
reader is waiting for a packet.

Because that's what it does.

We don't need two separate executors for long-running IO threads.

This should enable us to connect to contacts faster at startup and
whenever a new means of connecting becomes available.

Bug #47.

- fallback to SIM card before phone locale
- add disabled code to lookup the country from GPS, disabled because it requires a network call

- this will be useful later for e.g. auto-disabling Tor

This allows the oldest LocalAuthor to be used as the default.

All JVMs must support UTF-8 encoding.

We check for compatibility by specifying a schema version and minimum
schema version in the code, storing them in the DB, and checking whether
the DB is too old for the code or vice versa.

Dev task #52. Known but unverified identities are also supported, but
currently unused. These will be used in future for contacts who've been
introduced but not verified face to face.