Unfortunately, the Cassandra experiment has failed.
We've migrated the entire room objects data set on the live world, set up proper partitioning based on room key, and allocated server capacity to Cassandra instances 3x times larger in comparison to the MongoDB server.
The world simulation worked correctly. However, no considerable performance increase has been achieved.
This thread is now closed, the PTR is switched to the master branch. We're back at the world sharding idea, the discussion is in this thread.