- Always emit `ContactsChanged` from `contact::update_last_seen()` if a contact was seen recently
just for simplicity and symmetry with `RecentlySeenLoop::run()` which also may emit several events
for single contact.
- Fix sleep time calculation in `RecentlySeenLoop::run()` -- `now` must be updated on every
iteration, before the initial value was used every time which led to progressively long sleeps.
Add an event for a case if a multi-device synced config value changed. Maybe the app needs to
refresh smth on such an event. For uniformity it is emitted on the source device too. The value is
omitted, otherwise it would be logged which might not be good for privacy.
