Fix race between sync messages and receipts

Previously, when processing a backlog of sync messages and their
delivery receipts, we would fail to mark some messages as delivered even
though we got a receipt. This was due to an async race condition between
saving a sync message and fetching it after the receipt arrives.

Fix by re-ordering idb requests such that we save the message first and
fetch it after.

Fixes #479

// FREEBIE

js/models/messages.js

@@ -352,8 +352,8 @@
                     });
                 }
 
-                conversation.save().then(function() {
-                    message.save().then(function() {
+                message.save().then(function() {
+                    conversation.save().then(function() {
                         conversation.trigger('newmessage', message);
                         conversation.notify(message);
                     });