contacts-cache: return most relevant contacts

Return in the contacts in *reverse* rank order, i.e. the most relevant come first.
This is useful since we only want the first maxnum contacts, and those should of
course be the most relevant.

Update mu cfind/server as well.

cfind

lib/mu-contacts-cache.hh

@@ -136,16 +136,18 @@ public:
 	 * Prototype for a callable that receives a contact
 	 *
 	 * @param contact some contact
+	 *
+	 * @return to get more contacts; false otherwise
 	 */
-	using EachContactFunc = std::function<void(const Contact& contact_info)>;
+	using EachContactFunc = std::function<bool(const Contact& contact_info)>;
 
 	/**
-	 * Invoke some callable for each contact, in order of rank.
+	 * Invoke some callable for each contact, in _descending_ order of rank (i.e., the
+	 * highest ranked contacts come first).
 	 *
 	 * @param each_contact function invoked for each contact
-	 * @param max_num stop after at most so many contacts, or 0 for no limit
 	 */
-	void for_each(const EachContactFunc& each_contact, size_t max_num=0) const;
+	void for_each(const EachContactFunc& each_contact) const;
 
 private:
 	struct Private;