Maintain Referential Integrity while Deleting Resources

[alexanderkiel]

Currently, one can delete a resource even when it is referenced by other resources. We have to check referential integrity also at deletion.

However, because we didn't check for referential integrity while deleting in the past, we will add the property :check-refs to the transaction command delete. The default value will be false so that existing commands in the transaction log will continue to function. The value of :check-refs will be set depending on the env var ENFORCE_REFERENTIAL_INTEGRITY.