Add in value argument to throttle

[Pauan]

Right now the type of throttle is this:

fn throttle<A, B>(self, callback: B) -> Throttle<Self, A, B>
    where A: Future<Output = ()>,
          B: FnMut() -> A,
          Self: Sized

But it should be like this instead:

fn throttle<A, B>(self, callback: B) -> Throttle<Self, A, B>
    where A: Future<Output = ()>,
          B: FnMut(&Self::Item) -> A,
          Self: Sized