Asynchronous store interface

[lvh]

This used to be "deferred creation support" because that's where this problem originally showed up. In reality, it's a deeper problem: we don't have an abstracted store interface.

Right now, it is assumed that creation can be done synchronously: it happens right now, or it doesn't happen at all. In the first case, it returns 201 Created. It's possible that resource creation actually takes a while (ie returns a Deferred from the API ;-)), in which case it should say 202 Accepted.

The asynchronous store interface really just becomes most of the methods currently on ICollection.

[lvh]

This could work by using DeferredResource, but we really should have the asynchronous interface first.