Support If-modified-since for RawPage, fixes bug #10023.

Support If-modified-since for RawPage, fixes bug #10023.

The check to WikiPage::getTouched will load the latest revision data, which is
not ideal for performance. In a quick test using ab -c 100 -n 1000 and without
MemCached enabled, the request rate for plain GET requests drops slightly from
108 to 104 req/second. As described in my comment
https://bugzilla.wikimedia.org/show_bug.cgi?id=10023#c9, this could certainly be
improved on. Overall, a few unmodified responses should still make the
server-side performance impact of this patch positive, while improving
client-side performance considerably for unmodified content.