Process initial document state as incoming message.
This way it will be tracked as received from the remote server by y-websocket
and the update send to the server will exclude it.

Also use the sync services `opened` event inside WebsocketPolyfill
rather than mixing async `syncService.open().then` and event handling.
This allows us to emit `opened` and then `sync` from `syncService.open()`.

Signed-off-by: Max <[email protected]>

* Emit the documents last saved version in the push response
  if document state is send as well.
* Otherwise send the version as 0 so it cannot increase the sync service version.
* Use the versions returned by create and push requests alongside the document state
  to construct a consistent step with the last saved version that matches the document state.

This will result in the sync service version getting bumped
to the documents last saved version
once the coresponding document state has been processed.

Signed-off-by: Max <[email protected]>