Admission control plugin: Resource controller proposal

This proposal proposes a system for enforcing resource limits as part of admission control (see #2501)

I would like to get a resource standardized in the community that can enumerate resource constraints.

Would love some bike-shedding on the names, but I hope the core concepts are clear.  I have a functional prototype of the behavior described working that I continue to evolve, but would like to start submitting pull requests.

@erictune @smarterclayton @liggitt @jwforres