Add support for ordered relationships about ensembles

[drewmccormack]

This will be more difficult that it might first seem, which probably explains Apple's reluctance.

The problem is to come up with a set of deltas that when reapplied on devices that have stores in different states, leave them in the same state. For all other types of property, this is the case, but doing this for ordered data may be difficult.

[fdstevex]

Just noticed this, which may invalidate my bug concerning NSOrderedSet. Does this mean that until this is implemented, Ensembles simply won't work with models that use ordered sets?

[drewmccormack]

I don’t think it will work, no. It’s certainly untested. Sorry.

[drewmccormack]

What may work for this is simply storing the index of each inserted or moved object in the ordered set. When a conflict arises, we just perform the updates in order, resulting in a final set of indexes that are not guaranteed to be unique. We then sort deterministically, first on index, then on global id, and reindex to get final positions. This should be deterministic, leaving all devices consistent.

[drewmccormack]

Merged in 5ee1408