pagecache: Do not write 304s to the cache.

304s are triggered in reddit by the value of a request's
If-Modified-Since header. This header is not accounted for in the
request_key. We mustn't cache a response that isn't necessarily valid
for that request.
pagecache: Make actions able to specify pagecache policies.

This allows us to control pagecache behaviour at a per-action level.
There are three policy levels,

LOGGEDIN_AND_LOGGEDOUT - allow caching of resp. generated for loggedin.
LOGGEDOUT_ONLY - the current default.
NEVER - don't use the pagecache.
pagecache: Don't cache GET_info.

Most of these results are negative lookups, so we end up spending a lot
of memcache bandwidth and memory space on identical json responses.
pagecache: Add a debug header to indicate hits/misses.

To get the header in responses, be in debug mode or send
"Pragma: x-reddit-pagecache" in your request.