Convert constant to memoizing accessor. #4176.

The definition of `ORDER` indirectly referenced the code list cache.
This led to some situations where COMMUNICATION_RANK_CL1 was cached
as a non-list before it could be loaded into the database.
Merge branch '4176-code_list_cached_at_class_load' into develop
Log the number of codes loaded when successful.
Reset code list when using pg_dump, too.

This is already correctly handled for load_from_yaml.