Clarify that `id` must be consistent across all versions

[zimeon]

@awoods comment in OCFL/fixtures#87:

Should we add text around https://ocfl.io/draft/spec/#E037 along the lines of: This must be set in the first version of the object, must be unique in the local context, and must not change between versions of the same object.?

[zimeon]

I like the idea of clarifying the text that the id must be consistent across all versions. I don't think we should define it in terms of the first version however, because the v1 inventory may or may not stored and, for the case that all inventories are stored, I think typical validation strategy will be to start with the root/latest inventory and work backwards to check consistency

[awoods]

Updated wording (around https://ocfl.io/draft/spec/#E037) focused on consistency of object id across all "inventory.json" files at the object root and within version directories:

This must be unique in the local context and must not change between versions of the same object.