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

examples/companion_radio/MyMesh.cpp

@@ -862,6 +862,7 @@ void MyMesh::begin(bool has_display) {
 
   resetContacts();
   _store->loadContacts(this);
+  bootstrapRTCfromContacts();
   addChannel("Public", PUBLIC_GROUP_PSK); // pre-configure Andy's public channel
   _store->loadChannels(this);