[PERF-274] [PERF-317] Add NewRelic instrumentation to contentserver.

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.