- 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.
This dramatically increases the speed (7x in my tests) of the de-dupe
as google drive supports ListR directly and dedupe did not work with
`--fast-list`.
Fixes#2902