[EPIC] Add support for multiple collections from same schema

[AndrewSisley]

• Use interface syntax (consistent with view-schema only types) for schema-only declaration.
  • Name problem - interface cannot share same name as concrete type. We could prefix interface names with _?
• Anchoring is local only.
• It is a local choice if a node wishes to broadcast which collections a doc belongs to.
  • CollectionID/BroadcastID not required in order to have multiple cols locally. Existence of property will determine whether docs are broadcasted or not with the ID.
  • Branchable collection id will be the broadcast id if the collection is branchable, otherwise the property may be set to anything by the user (we could limit this to ID types, but why?) (SDL, PatchCollection).
  • Later, we can offer to generate an id.
• Allows P2P linking to remain local - nodes subscribe to a col id, and store it against their chosen local RootID.
  • Need to define behaviour for linking. For now, add to all by default, unless broadcasted id matches a local id.
• Need to decide what happens when adding new collections (datastore migration?)
• Long term we can optimise storage space for docs existing in multiple cols, but for now keep things simple and duplicate - it gets a little fiddly with Lens.
• Introduce means to remove doc from collection without deleting it.
• Introduce means to split doc and diverge it in different collections.
• Introduce means to create doc in multiple collections (via GQL and other clients)
• Handle the __type GQL field - it has an interfaces prop that needs to return set of interfaces.

[AndrewSisley]

Do not work on this issue for now. We are probably removing schemas as a public concept in the medium term.