reactore-core: Expose the deleage on the ReplyToConsumer.

Exposing the delegate allows to get a reference to the enclosed
function after it has been added. This enables use-cases like
cancelling the registration and then calling a method on the
function (like shutdown).
Merge pull request #203 from daschl/reactor-enclosed-function

reactore-core: Expose the deleage on the ReplyToConsumer.