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
- Change rclone/fs interfaces to accept context.Context
- Update interface implementations to use context.Context
- Change top level usage to propagate context to lover level functions
Context propagation is needed for stopping transfers and passing other
request-scoped values.
- make Close permanent and return errors afterwards
- use RangeSeek from the wrapped reader if present
- add a limit to chunk growth
- correct RangeSeek interface behavior
- add tests
