(bug 1072291) revert pushlog caching strategy

The pushlog cache now uses the top revision of the last push.
Also, increase the time limit to fetch the pushlog to 3 minutes
edmorley raises a good point here, https://bugzilla.mozilla.org/show_bug.cgi?id=1072291#c4. I think we need to return the response from ``JsonExtractorMixin.extract()`` and if it's a ``404`` repeat the ``extract()`` call without the ``fromchange`` parameter to account for the repository reset case.