Simplify some of the presence code by reducing duplicated code between
worker & non-worker modes.
The main change is to push some of the logic from `user_syncing` into
`set_state`. This is done by passing whether the user is setting the presence
via a `/sync` with a new `is_sync` flag to `set_state`. If this is `true` some
additional logic is performed:
* Don't override `busy` presence.
* Update the `last_user_sync_ts`.
* Never update the status message.
