Add explicit message field for locally_echoed.

We now set locally_echoed to true for messages that are
locally echoed, and we change some of our code to look
for this flag.

static/js/message_flags.js

@@ -11,7 +11,7 @@ function batched_updater(flag, op, immediate) {
     function server_request() {
         // Wait for server IDs before sending flags
         var real_msgs = _.filter(queue, function (msg) {
-            return msg.local_id === undefined;
+            return !msg.locally_echoed;
         });
         var real_msg_ids = _.map(real_msgs, function (msg) {
             return msg.id;