The waker stored in the future will only wake up and re-poll the stream
if the call to future returned Poll::Pending. Because of this, if we
didn't receive enough data to reconstruct the ResourceDiff, we need to
loop and poll the inner future again rather than return Poll::Pending
ourselves.
Rather than returning a Receiver<Bytes>, instead return a ResourceStream
struct that implements the Iterator trait so callers can simply iterate
over the ResourceDiff items received.
Cecylia Bocovich