Payload to connect related activities

[sfolsom]

While important steps in entity management, we didn't have a clear direction for how to handle more complicated scenarios, e.g. partial splits, partial merges. We will defer this to a future round of specification development. The spec, as it is written, provisions for full Splits and Merges using a combination of Deprecate and Create. For partial splits/merges (where the existing entity continues to exist), Update and Create can be used. The specification provides RDF Patch as an option making clear how entities are related; without RDF Patch the consumer must dereference the entities in question and make sense of the changes. In the future we may want to surface these types of changes and related activities in a normalized way (regardless of how they are modeled in the entity RDF) with "useInstead" and "seeAlso" like semantics consistently reflected in the ActivityStream.