This patch adds a bucket backend for the core media cache that interfaces to a
remote HTTP server. When a media item is requested in the cache, the cache will
query its bucket backends to see if they can provide the media item. If that
media item has a scheme of HTTP or HTTPS, this backend will be invoked.

The backend provides callbacks for the following:
 * create - this will always retrieve the URI specified by the provided
            bucket_file, and store it in the file specified by the object.
 * retrieve - this will pull the URI specified and store it in a temporary
              file. It is then up to the media cache to move/rename this file
              if desired.
 * delete - destroys the file associated with the bucket_file.
 * stale - if the bucket_file has expired, based on received HTTP headers from
           the remote server, or if the ETag on the server no longer matches
           the ETag stored on the bucket_file, the resource is determined to be
           stale.

Note that the backend respects the ETag, Expires, and Cache-Control headers
provided by the HTTP server it is querying.

ASTERISK-25654

Change-Id: Ie201c2b34cafc0c90a7ee18d7c8359afaccc5250