bug search cache makes next/prev return old data

Symptoms
========

Clicking next or prev on a new style bug listing will return old data - even if you e.g. triage lots of bugs they will still show up until you refresh the page. This is a significant difference from the non-ajax behaviour and not consistent with other AJAX applications like e.g. gmail. (But it is consistent with e.g. twitter and other sites that show read-only batches).

Cause
=====

We have a batch cache meant to reduce latency but it has no expiry / replacement policy.

Possible solutions
==============

* Remove the cache entirely

* Update the current batch asynchronously and show a transition on added/removed rows.