feature request: upstream close propagation

[mccraigmccraig]

i wanted to have a bunch of sources all transitively close automatically when their ultimate sink was closed

as it stands :upstream? true only seems to work on upstream sources connected directly with (connect ... {:upstream? true}) and any map operations in between source and sink will stop close propagation. additionally upstream sources are only closed when items are flowed to the sink marked with :upstream? true, rather than when the sink is closed

i gather from #85 that there are some difficulties with upstream navigation so this feature request may not be feasible, but it would sure be nice :)

[ztellman]

I've got parts of this implemented, I'll take another look and see what I can do. Thanks for adding your vote.