Resolve conflicts in series

Attempting to resolve outgoing conflicts in parallel triggers multiple
requests for new keys from the server and causes it to return a 500
error.

js/models/conversations.js

@@ -273,11 +273,15 @@
 
         return textsecure.storage.axolotl.removeIdentityKey(number).then(function() {
             return textsecure.storage.axolotl.putIdentityKey(number, identityKey).then(function() {
+                var promise = Promise.resolve();
                 this.messageCollection.each(function(message) {
                     if (message.hasKeyConflict(number)) {
-                        message.resolveConflict(number);
+                        promise = promise.then(function() {
+                            return message.resolveConflict(number);
+                        });
                     }
                 });
+                return promise;
             }.bind(this));
         }.bind(this));
     },

js/models/messages.js

@@ -121,6 +121,7 @@
                         this.save({sent: true, errors: errors});
                     }.bind(this));
                 }
+                return promise;
             }
         },
         handleDataMessage: function(dataMessage) {