Support progress-based timeouts for downloads

It is hard to define an end-to-end timeout for downloads as (1) the size varies from 0 bytes to 5TiB, (2) part of the delay is caused by the application pulling data fast enough.

We need to define some kind of "progress based" (or rather "lack of progress based") timeouts. The request should timeout if we were not able to receive at least X bytes per second. We should use a different timeout value for "the operation should start downloading within X milliseconds".