No Identifier for ApiProvisioner on Delete/Expunge

When CoApiProvisionerTarget::deletePerson() is called, it tries to find the identifier to send to the endpoint. It does this by examining $provisioningData, which doesn't exist in this context. By the time expunge triggers provisioning, the identifier has been deleted from the record.

ApiProvisioner needs to either track the provisioned identifier in a lookaside table that doesn't get cleared on expunge, or maybe in the provisioning status record (if that survives expunge), or maybe trawl changelog data to find the identifier in deleted records.