This reverts commit 09586d8a.

This includes the course key/path for a given contentserver request,
whether or not the asset is locked, cacheable, has been modified, etc.

This should give us some better insight as to what sort of requests are
coming into the contentserver to figure out whether or use of a CDN in
front of these assets is as efficient as effective as it could be.

This introduces a mechanism to control the time-to-live for an unlocked
course asset, which will allow browsers and intermediate proxies/caches
to cache these course assets, determinstically.

Locked assets, with their nature of requiring authorization, are not
eligible for caching.