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.

Generate a random UUID instead of using a fixed UUID. Close sockets when
exceptions are thrown (not doing so can cause problems with subsequent
sockets on Android). Use a semaphore with tryAcquire() instead of a lock
when making alien calls, to avoid possible deadlocks.