Track user data for entered chat rooms.
Add debug build rules.
Add and use convenience function Batyr_db.use_accounted.
Adjust use of options and Batyr_db.use.

  - Batyr_data: Let Node.cached_id and Muc_room.of_node return option.
  - Batyr_data, Batyr_presence: Inform about room in debug msg.
  - Move code out of Batyr_db.use where possible.
Add Cache_by_physical_bijection, adjust cache add.

  - Batyr_cache: Let cache add predictably retain the existing entry.
  - Batyr_cache.Cache_by_physical_bijection: Alternative using physical
    equality.
  - Use it for Node_cache in Batyr_data.