Add function req_perform_promise (fixes #501)

[gergness]

An implementation of req_perform_promise() - #501

It has worked pretty well in my initial testing, though the way I'm tracking which pools already have a later task polling them does feel a little fragile.

[hadley]

I added a couple of small comments but I'll try and find someone who knows more about promises than me to take a deeper look.

[jcheng5]

Impressive work, thank you!

[hadley]

Thanks @gergness!