Add a new flag to disable users. This can be useful to temporarily
deactivate an account without erasing data.
The user goroutine is kept alive for simplicity's sake. Most of the
infrastructure assumes that each user always has a running goroutine.
A disabled user's goroutine is responsible for sending back an error
to downstream connections, and listening for potential events to
re-enable the account.
This is a mecanical change, which just lifts up the context.TODO()
calls from inside the DB implementations to the callers.
Future work involves properly wiring up the contexts when it makes
sense.
