Fix realm issues

[bjoerge]

This fixes two issues:

• 1. A request to checkpoint at a domain with no associated realm will now return a 412 (Precondition failed), e.g.:

$ curl -I http://this.domain.is.valid.but.has.no.realm/api/checkpoint/v1/identities/me
HTTP/1.1 412 Precondition Failed
No realm is associated with the domain 'this.domain.is.valid.but.has.no.realm'

• 2. A request for an identity id that is in another realm than the realm associated with the request host will not be returned. Instead /identities/me will return {} as if the identity doesn't exists at all.