WaitIterator: don't re-use _running_future

WaitIterator: don't re-use _running_future

When used with asyncio.Future, WaitIterator may skip indices in some
cases. This is caused by multiple _return_result calls after another,
without having the chain_future call finish in between. This is fixed
here by not hanging on to the _running_future anymore, which forces
subsequent _return_result calls to add to _finished, instead of causing
the previous result to be silently dropped.

Fixes #2034