Log requests served from the API cache

There is no logging of requests successfully served from the API cache.
Typically threadedhttp logs the request, and api logs the response.

This change logs when CachedRequest does not need to submit the request,
logging the request parameters and the filename containing the cached data.

Change-Id: Ib4aea6b5d401121fedb2ae91b8869b1872d1b4db
Merge "Log requests served from the API cache"