context.Context friendly API

[harry-xm]

https://github.com/matryer/vice/blob/660007f4486b289078664b182d1ecd2a22b29b02/transport.go#L24

Done() in context.Context returns a <-chan struct{}, which is not compatible with vice.Transport.

    *Transport does not implement vice.Transport (wrong type for Done method)
        have Done() <-chan struct {}
        want Done() chan struct {}