The invitation and private messaging UIs are currently broken. Some key
rotation bugs were fixed; others may have been created (unit tests
needed). An encoding for private keys was added. Pseudonyms were moved
out of the messaging package and ratings were moved in.

The UI may access the DB in response to UI or DB events; to maintain a
consistent view of the DB's contents, the tasks performing these
accesses must be prevented from overlapping, and must produce consistent
results if reordered. A single-threaded executor and latches are used to
prevent tasks from overlapping, without blocking non-UI access to the
DB.

Fields in Android UI objects that are accessed from background threads
must be declared volatile. UI objects use data attached to DB events to
avoid DB lookups, which complicates the UI code but should improve
performance.

Rejecting codes greater than 2^19 - 1 creates a confusing user
experience.

Only the bundle contents created by Briar classes are encrypted.

This limit is necessary to limit the size of subscription update
packets; it can be lifted when incremental subscription updates are
implemented.