- 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.
Before this change serving bucket based objects
`[remote:bucket]/path/to/object` would fail with 404 not found.
This was because the leading `/` in `/path/to/object` was being passed
to NewObject.
If `--rc-user` or `--rc-pass` is set then the URL that is opened with
`--rc-files` will have the authorization in the URL in the
`http://user:pass@localhost/` style.
