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.

The new format is simpler but less efficient for small integers, short
strings and short byte arrays.

Note that this is only the generator part of Fortuna, not the
accumulator. The generator requires a seed, which is provided by a
platform-specific implementation of SeedProvider. On Linux the
implementation reads the seed from /dev/urandom.

If the schema of the database is incompatible with the schema expected
by the code, the database throws a DbSchemaException. LifecycleManager
indicates the error to BriarService, which uses HomeScreenActivity to
show a notification and quit the app.

Offered and requested message IDs are stored in the database rather than
being owned by DuplexConnections. This paves the way for moving to a
simplex transport layer.