Before this change calling core/command gave the error
error: response object is required expecting *http.ResponseWriter value for key "_response" (was *http.response)
This was because the http.ResponseWriter is an interface not an object.
Removing the `*` fixes the problem.
This also indicates that this bit of code wasn't properly tested.
- 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.
