fix(examples): Fix tower examples

[davidpdrsn]

Doing

let clone = self.inner.clone();
Box::pin(async move {
    let response = clone.call(request).await?;
    Ok(response)
})

If self.inner is (or contains) a tower::buffer::Buffer might panic.

That is because cloning a Buffer drops the permit that was acquired in
poll_ready, meaning it is no longer ready and panic in call.

The solution is to use mem::replace to take the ready service and pass
that into the async block.

Fixes #545

[davidpdrsn]

This is basically #547 with my suggestions applied. OP was inactive.