bootstrap RTC from contact.lastmod and improve slot overwrite logic

slot overwrite logic can now safely use contact.lastmod to find oldest contact for overwrite

src/helpers/BaseChatMesh.h

@@ -88,6 +88,7 @@ protected:
     memset(connections, 0, sizeof(connections));
   }
 
+  void bootstrapRTCfromContacts();
   void resetContacts() { num_contacts = 0; }
   void populateContactFromAdvert(ContactInfo& ci, const mesh::Identity& id, const AdvertDataParser& parser, uint32_t timestamp);
   ContactInfo* allocateContactSlot(); // helper to find slot for new contact