Refactor the scheduler out of the JobTracker

First I would like warn you that my proposition is assumed to be very naive. I just hope that reading it won't make you lose time.

The aim

It seems to me that improving Hadoop scheduling could be very profitable. But, it is hard to implement and compare schedulers, because the scheduling logic is mixed within the rest of the JobTracker.
This bug is the first step of an attempt to improve the Hadoop scheduler. It re-implements the current scheduling algorithm in a separate class called JobScheduler. This new class is instantiated in the JobTracker.

Bug fixed as a side effects

This patch probably cannot be submited as it is.
A first difficulty is that it does not have exactly the same behaviour than the current JobTracker. More precisely, it doesn't re-implement things like code that seems to be never called or concurency problems.
I wrote TOCONFIRM where my proposition differ from the current implementation, so you can find them easily.
I know that fixing bugs silently is bad. So, independently of what you decide about this patch, I will open issues for bugs that you confirm.

Other side effects

Another side effect of this patch is to add documentation about each step of the scheduling. I hope that it will help future improvement by lowering the level required to contribute to the scheduler.
It also reduces the complexity and the granularity of the JobTracker (making it more parallel).

The future

If you feel that this is a step the right direction, I will try to propose a JobSchedulerInterface that many JobSchedulers could implement and to propose alternatives to the current « FifoJobScheduler ». If some of you have ideas about that please tell ^^ I will also open issues for things marked as FIXME in the patch.