Implement the scheduler for controling parallel segment uploads

[iglesiasbrandon]

We want to use available bandwidth while not over-saturating. One way to tell when your upload limited is when all of your individual piece uploads start taking the same amount of time (assuming all of the pieces are the same size). When that begins to happen, you should limit the concurrency some. When you have some pieces completing faster than others, you should increase the concurrency some.

Deliverable: Implement an initial pass at a scheduler that does this and evaluate.

[iglesiasbrandon]

@zeebo would we be able to estimate what sort of impact this might have on slow connections, average connections, and fast connections? how could we simulate this?

[kristaxox]

@zeebo poke for story point estimation :)

[mniewrzal]

@zeebo are there any code changes for this already?