I think there are two factors to consider, when the entity is marked as
Immutable, and when the EntityEntry can be shared between sessions.
Our use case concerns the ImmutableEntityEntry being cached and shared
between sessions, so we need to check if the Entity is reference cached
in the 2lc.
The first part of the cassandra backend work is ready for review. I've
backed out the incomplete association support, leaving the basic
framework for the backend module with enough functionality to support
entities that don't have relations. It's still larger than an average
PR, but the module
