From a Rob Pike pass over the tree:
- singleOnce adds deltas to d.completed like pump does, instead of
storing absolute offsets, so the counter has one write discipline.
- one overMirrors helper replaces the three copy-pasted mirror-fallover
loops in probeRetry, fetchSeg and single.
- failedDownload becomes a pointer, the only value-type Download impl, so
Engine.untrack's == is always pointer identity and can't compare the
fields of a value (keying on Stat().ID is out — a torrent's ID changes
from source to infohash once metadata loads).
