JSON CRDT `.fromBinary()` improvements

[streamich]

• [x] Add ability to set local session ID, after loading model from the server, Model.setSid().
• [x] Reconsider .setSchema() on old documents.
• [x] Add ability to lightly switch model session ID, say .setSid(123).
  • [x] Or, add ability to set schema with SESSION.GLOBAL, while preserving the original session ID.

Consider adding a new static .load() method, which simplifies

const model = Model
  .fromBinary(blob)
  .setSchema(schema)
  .fork(sid);

to

const model = Model.load(blob, sid, schema);