[tsgfddzw] Data URI cache

Extend the Data URI scheme to permit caching and skipping over cached content.

I think this can be done without modifying the specification.  We need a MIME type which encapsulates additional metadata and inner data (itself described with another MIME type).  The metadata includes the size and the content hash and is encoded early in the outer MIME stream.  With these two pieces of information, the client can use HTTP byte range requests to skip over content it already has cached.  This might require terminating a TCP stream in progress.