Write spec for Sauna Entity Server

[alexanderdean]

Key questions:

• Do we store the s-d JSONs as first-class citizens in the underlying storage, or just as strings?
• Do we need validation on PUT (I think yes)?
• Do we need to allow partial updates (e.g. update the isChurnRisk property in a user entity only)?
• Do we just start with ES as the backing store - if so, HTTP client?
• Are we key agnostic, or do we support structured keys? i.e. does the entity server have any semantic understanding of what the key represents?
• How RESTful are we? Do we expect you to POST to an entity-specific path, or do we just let you post an s-d JSON and then the Entity Server figures out which entity it relates to? What should the path be for GETs? Maybe entities/com.acme/user/jsonschema/1/{id}?

[alexanderdean]

For structured keys - we could do something neat where when you PUT, you note on the querystring which field(s) you want to use as the key. Then the GET could be:

entities/com.acme/user/jsonschema/1/userId:{id}