[libcontacts] Prioritize requests that make app startup more responsive

This PR changes the loading logic in seasidecache in order to make RecentContactsList load much faster. Before the PR, it would wait for the entire cache to be populated before resolving the recent contacts. With this PR, the address resolution gets priority and is handled first.

Tested with jolla-contacts and 10k contacts in the database, it now loads the recent contacts in about 1 second rather than 5. It's still busy loading contacts in to the cache for a while after that, but at least the recent contacts are immediately usable.
Not a bug, but poor design perhaps.  We always fetch the constituents from GUI on a contact update of a displayed contact, so the data will be updated.  It would make sense for it to be correctly handled by the cache independently of the client's actions.