OF-2499: No longer serialize MUC history with room
As suggested in a 'TODO' comment in code, this replaces the MUC history that is stored inline with each MUC room, with a cache lookup.
This change intends to result in a performance boost, as retrieving/storing MUC rooms from a cache (and serializing them between cluster nodes) no longer includes the history. The data associated with history is typically very large (~50kb for 25 messages), but changes a lot less often than the room itself.
