use session.cancel in the router to make blocking safe, reduce size of fromRouter buffer so the drops in the switch are closer to the intended front-drop behavior

src/yggdrasil/router.go

@@ -166,8 +166,8 @@ func (r *router) handleTraffic(packet []byte) {
 		return
 	}
 	select {
-	case sinfo.fromRouter <- &p: // FIXME ideally this should be front drop
-	default:
+	case sinfo.fromRouter <- &p:
+	case <-sinfo.cancel.Finished():
 		util.PutBytes(p.Payload)
 	}
 }