Serialize sending and adding messages to a convo

Previously, if a message was sent in between the receive time of an
incoming message and the time it is actually added to the conversation's
message collection (which only occurs later after several async
callbacks), the incoming message would be inserted not-at-the-end of the
collection since it is ordered by receive time. This tricked the front
end into assuming the message was an older message instead of a new one.

Fixes #490

// FREEBIE

js/models/conversations.js

@@ -112,7 +112,21 @@
         this.set({tokens: tokens});
     },
 
+    queueJob: function(callback) {
+        var previous = this.pending || Promise.resolve();
+        var current = this.pending = previous.then(callback, callback);
+
+        current.then(function() {
+            if (this.pending === current) {
+                delete this.pending;
+            }
+        }.bind(this));
+
+        return current;
+    },
+
     sendMessage: function(body, attachments) {
+        this.queueJob(function() {
             var now = Date.now();
             var message = this.messageCollection.add({
                 body           : body,
@@ -142,6 +156,7 @@
                 sendFunc = textsecure.messaging.sendMessageToGroup;
             }
             message.send(sendFunc(this.get('id'), body, attachments, now));
+        }.bind(this));
     },
 
     isSearchable: function() {

js/models/messages.js

@@ -267,6 +267,8 @@
                 conversationId = dataMessage.group.id;
             }
             var conversation = ConversationController.create({id: conversationId});
+            conversation.queueJob(function() {
+                return new Promise(function(resolve) {
                     conversation.fetch().always(function() {
                         var now = new Date().getTime();
                         var attributes = { type: 'private' };
@@ -353,6 +355,9 @@
                             conversation.save().then(function() {
                                 conversation.trigger('newmessage', message);
                                 conversation.notify(message);
+                                resolve();
+                            });
+                        });
                     });
                 });
             });