Discovery add history

[erikerikson]

Fixes #699

Proposed Changes

• Add discovery endpoint as a Service entity by which it publishes changes to itself
• Add historical querying capability (perhaps better delivered by allowing subscriptions to specify delivery of events starting from those available as of a time in the past)

Release Note

Add Discovery Endpoint as a "self" Service Entity
Add historical service change query endpoint 

[erikerikson]

Having a requestBody on a delete is discouraged by OpenAPI for reasons discussed here. To abusively summarize the most useful piece, there is no standard about the interaction of HTTP DELETEs on to resource collections and the caching layers which may cause undesirable lack of caching on one hand and/or a use of cached resources inappropriately on the other.

e.g.

DELETE /services

[
  { id: {id_0} },
  ...
  { id: {id_N} },
]

Does not clear /services/{id_0} from HTTP response caches which, unless configured not to cache, are allowed to cache and return that value following the resource's deletion using a DELETE with a request body.

[erikerikson]

@duglin this has been pending the AIs from Clemens and Klaus but I am wondering if we expect those to alter this in some way? If not, we might be able to merge this and proceed by altering it as necessary?

[duglin]

Approved on the 10/14 call