Use Evennia cache through SharedMemoryModel

[vincent-lg]

Using evennia.utils.idmapper.models.SharedMemoryModel instead of django.db.models.Model will allow to benefit from the Evennia cache. Instead of hitting the database (once or twice per view, depending on the status of the root page), Evennia will cache the requested page so a database hit per page is only necessary if the page isn't already cached in memory.

This would require to change all field names (field to db_field). That would require a major migration, but one that might be very useful in the end.