Add a flatten strategy to limit concurrency

[mdiep]

concat limits concurrency to 1 signal. merge allows unlimited concurrency. It'd be nice to have something in-between.

ReactiveObjC has this as flatten:.

This is useful for things like Carthage/Carthage#1841.

[NachoSoto]

Or change .merge to .merge(maxConcurrency: UInt) with static let merge: FlattenStrategy = .Merge(UInt.max).

[mdiep]

And static let concat = .merge(1)?

[NachoSoto]

As long as it's not less performance, merging (no pun intended) the implementations would be amazing.

[andersio]

I'd call it thebound or concurrent strategy. 🙂

[andersio]

Implemented in #298.