Make the parser cache expire at 30 days again

Per bug 44570 comment 32. Doesn't fix the bug, just brings the parser
cache expiry into line with the max age sent to purgeParserCache.php
and, apparently, available disk space. Also avoids a massive CPU spike
occuring next time purgeParserCache.php runs. Some cleanup of the
parsercache database is still necessary, to adjust the expiry time of
objects expiring in 2014, so that they can be removed by
purgeParserCache.php.

Change-Id: I61a706d931ff2e53108c082da88fa91b82ea1214