Before this change, if one connection was authenticating this would
block any others from authenticating.
This was due to ssh.NewServerConn not being called in a go routine
after the Accept call.
This is fixed by running the ssh authentication in a go routine.
Thanks to @FiloSottile for advice on how to fix this.
See: https://github.com/golang/go/issues/43521
Before this change the sftp handler returned a nil error for unknown
operations which meant the server crashed when one was encountered.
In particular the "Readlink" operations was causing problems.
After this change the handler returns ErrSshFxOpUnsupported which
signals to the remote end that we don't support that operation.
See: https://forum.rclone.org/t/rclone-serve-sftp-not-working-in-windows/12209
- 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.
