Remove Old Feedback signature in favour of new.

[sergdort]

At this point I do not see a value having two sightly different implementation of the same thing.

I proposing to remove old system operator that takes scheduler in favour of new queue-drain event processing.

FeedbackLoop.Feedback -> Feedback

[andersio]

We could have them staying a bit longer, just so existing users dont need to migrate everything all together.

#7 makes FeedbackLoop.Feedback terser as Loop.Feedback.