Load document content lazily in YRooms

[dlqqq]

The YRoom should only load the document content after a Websocket connection is made or if a Python consumer calls get_ydoc() / get_jupyter_ydoc() on a room.

With #117, the server memory use is bounded to the file size of all rooms opened since the server started, plus the YDoc history of any active rooms. Even though #117 restarts inactive rooms, it still loads the file content for each inactive room after restarting.

Fixing this issue would lower that bound even further to only contain the file size + YDoc history of active rooms. Inactive rooms would only reserve a mostly-empty YRoom instance that should have negligible performance/memory impact.