Callback for FastScroll state

[brianwernick]

Currently event when the user is scrolling very quickly every view needs to be bound (because we don't know the scroll velocity). It would be nice to have a callback informing us when the RecyclerView is being scrolled quickly and exits being scrolled quickly (i.e. after a specified velocity the event is triggered).

This would allow us to not-rebind views when quickly scrolling (allowing for performance optimizations)