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.

Note that some devices (I'm looking at you, Huawei U8110) misreport
connectivity by failing to set EXTRA_NO_CONNECTIVITY and returning true
from ConnectivityManager.getActiveNetworkInfo().isConnected() when
there's no connectivity.

The latest version of libevent (2.0.21-stable) works fine.

Fixes bug #7. The service is restarted after a crash even if
onStartCommand() returns START_NOT_STICKY, but after restarting the DB
key is not restored and so the app can't usefully proceed.

If the app restarts after crashing and tries to return to the activity
where it crashed, DatabaseConfig's encryption key will be null and no DB
operations can be performed. It's better to crash again in that case
than for all DB operations to fail.

Fixing bug #3 will make this irrelevant.

Also fixed a bug in addGroup(): SELECT COUNT (NULL) doesn't work.

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.