Identities should no longer come from the System Shard

(Bug, To be Fixed , Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Users and Identity
Summary: Now that we are potentially giving out Identities like water (especially in Shared Links / Open Invitations), we need to not be consuming the limited resource of ShardKind.System for them. They should come from the modern OID generators instead.
Depends Upon:
The complication here is that this means we need to route the request for OIDs remotely. Which means that the relevant calls in UserPersistence need to become Future-oriented. I am opening a separate Issue for that.