fix: Fix emitting ContactsChanged events on "recently seen" status change (#5377)

- 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.

src/events/payload.rs

@@ -182,7 +182,7 @@ pub enum EventType {
         timer: EphemeralTimer,
     },
 
-    /// Contact(s) created, renamed, blocked or deleted.
+    /// Contact(s) created, renamed, blocked, deleted or changed their "recently seen" status.
     ///
     /// @param data1 (int) If set, this is the contact_id of an added contact that should be selected.
     ContactsChanged(Option<ContactId>),