Design versioning features

[peacekeeper]

There is consensus that it should be possible to resolve earlier versions of a DID Document, in order to be able to validate proofs, authentication events, etc. at an earlier date. This raises a few topics:

1. When resolving a DID, the output data (DID Document and/or metadata) could include a version identifier and/or timestamps of last update, etc.
2. What if anything at all do we want to say about DID Document version identifiers (method-specific? could be sequential number, UUID, etc.).
3. We should define one or more input parameters for DID Resolution that request a DID Document a) with a given version identifier, or b) at a certain timestamp.
4. Note that while all DID methods MUST support updates, there is no requirement for DID methods to keep all previous DID Document versions, so this may be a method-specific feature.

Thoughts?