Resolve conflicts one at a time

Previously, with a mix of text and media messages in conflict,
asynchronous callbacks aligned so as to fail to remove some of
the conflict objects on messages.

Fix by serializing conflict processing, but making sure to move
on through the message list even if some conflict resolutions fail.

Fixes #370

// FREEBIE

js/models/messages.js

@@ -199,7 +199,6 @@
         resolveConflict: function(number) {
             var error = this.getKeyConflict(number);
             if (error) {
-                this.removeConflictFor(number);
                 var promise = new textsecure.ReplayableError(error).replay();
                 if (this.isIncoming()) {
                     promise = promise.then(function(dataMessage) {
@@ -207,6 +206,7 @@
                     }.bind(this));
                 } else {
                     promise = promise.then(function() {
+                        this.removeConflictFor(number);
                         this.save();
                     }.bind(this));
                 }