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.

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.

The distinction between inbox groups and other groups can be maintained
internally, there's no need to represent it on the wire.

This can happen on Android if the app is automatically restarted after a
crash.

Unit tests would've caught this.

Private messages are now the same as group messages, but groups can be
private or public. When a contact is added, a private group is created
and designated as the inbox for exchanging private messages with the
contact.