Closing a stream in the parallel chunked reader cancels the stream's
context, so the in-flight read returns context.Canceled. This was wrapped
and returned as "failed to read stream", which the VFS cache downloader
treats as a real download error - it only recognises
asyncreader.ErrorStreamAbandoned as a benign teardown, as returned by the
sequential reader.
Return asyncreader.ErrorStreamAbandoned for a cancellation so tearing down
the parallel reader (on close, seek or reposition) is recognised as benign,
matching the sequential reader, instead of logging download errors and
retrying when --vfs-read-chunk-streams is used with --vfs-cache-mode full.
This converts the ChunkedReader into an interface and provides two
implementations one sequential and one parallel.
This can be used to improve the performance of the VFS on high
bandwidth or high latency links.
Fixes#4760
Nick Craig-Wood