ZTypeRegistry::SELF_REFERENTIAL_KEYS was introduced to allow validation of ZObjects that have references to their own IDs (e.g. ZPersistentObject or ZType...) without having "Zid not known" errors.

With current validation system, this might not be necessary. At least it doesn't seem to be used anymore, so we should:

• Check if self-references raise errors
• If they don't, remove SELF_REFERENTIAL_KEYS
• If they do, figure out how to avoid these