Support HTTP range requests.

[jevolk]

Supporting HTTP range requests foremost allows efficient transfer of large media on behalf of clients for the /media/ endpoints. Construct conducts downloads of files from remote servers synchronously on request of a client; if the client goes away, the request is interrupted and the result is a partial download. We can do better by continuing the remote download at an offset when the client retries the request, rather than starting over from scratch.

There are probably other gimmicks we can accomplish with range requests as well.