Parallel functionality

[Patrick Smith]

A bit like flat mapping an array

Instead of:

mutating func updateOrDeferNext() -> Deferred<HTTPRequestProgression>? {}

have:

mutating func updateOrDeferNext() -> [Deferred<HTTPRequestProgression>] {}

This means the finishing state is [] instead of nil

Also allows parallel processing quite simply. Would not cover if you wanted to later 'join' the parallel processes later.