A new idea that removes the extra box of OperatorFuture

[Xuanwo]

We can store the future in OperatorFuture directly instead of Box<dyn Future>. And than, return FutureStat<impl Future<Output = Result<Metadata>>> instead.

No need for Box::pin() now!

Drawback is users can't store FutureStat anymore unless they use Box::pin(), and it's a breaking change.