Discuss how the Fetch implementation could support streaming

[purrgrammer]

From the gitter channel:

I like the deduplication feature, but when I run a fetch I'd like to be able to get the data as it becomes available. (Imagine I have a websocket and I want to push data through it as it becomes available.) My issue is that if I use:
(fetchA,fetchB).tupled even if the two fetches ultimately execute in parallel, I have to wait until they both complete to get the full tuple.