Inline edit shows older version of issue if edit responses are received out of order

Found on v5.1-kickass9#757-sha1:a700ce2. If a user updates 2 single fields and the responses are received in reverse order it looks like update 2 never happened.

Steps to reproduce:

• Open up Charles proxy and set a breakpoint on the URL that the inline edit uses (IIRC it's /EditAction)
• Open an issue and edit its Priority. Click the checkbox. The request will show up in Charles's breakpoint view.
• On the same issue, type in a comment and click "Add". The 2nd request will show up in Charles' breakpoint view. Click Execute on this 2nd request. The comment appears on your issue.
• Now click Execute on the 1st request. The changed Priority appears on the issue, but the comment disappears.
• Reload the page and both the changed Priority and the added Comment are visible.

Network latency can cause this to happen in real world usage. We should implement inline edit in such a way that the order of received responses does not affect what eventually gets shown (after all responses are in).

cc jcreenaune, sharwood