Do app -> client communication more like tfrpc so that it's easier to get responses.

git-svn-id: https://www.unprompted.com/svn/projects/tildefriends/trunk@3956 ed5197a5-7fde-0310-b194-c3ffbd925b24

core/tfrpc.js

@@ -14,6 +14,9 @@ if (k_is_browser) {
 function make_rpc(target, prop, receiver) {
 	return function() {
 		let id = g_next_id++;
+		while (!id || g_calls[id]) {
+			id = g_next_id++;
+		}
 		let promise = new Promise(function(resolve, reject) {
 			g_calls[id] = {resolve: resolve, reject: reject};
 		});