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

Of course, not all devices support Bluetooth...

Hopefully this gives users a better understanding of whether their
messages and contacts are accessible than "Briar is running" and "Quit".

GroupActivity needs to receive a similar treatment.

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.