Currently we support this workflow:

1. client 1 adds document
2. client 2 obtains document

but sometimes, the client want to obtain the document by known id and get it initialized later

1. client 2 obtains document (because it didn't exist, it is initiated to null)
2. client 1 initiates document (rewrites null with other ID)
3. client 2 get a content synchronized

I believe this should be implemented in the server so that get-or-create-document is one atomic from client's viewpoint.