DownloadNode: fix lost-progress in fetch_failed, tolerate cancel when no segment...

DownloadNode: fix lost-progress in fetch_failed, tolerate cancel when no segment-fetch is active. Fixes #1154.

The lost-progress bug occurred when two simultanous read() calls fetched
different segments, and the first one failed (due to corruption, or the other
bugs in #1154): the second read() would never complete. While in this state,
cancelling the second read by having its consumer call stopProducing) would
trigger the cancel-intolerance bug. Finally, in downloader.node.Cancel,
prevent late cancels by adding an 'active' flag