It should not be possible to override CLAIM_DEF for existing schema-did pair

We should have an assumption to be able to have just one CLAIM_DEF (that is a key) for an ISSUER_DID.
If one needs to rotate a key, a new DID must be used. It means that ISSUER_DID is rather a CLAIM_DEF's DID (or key's DID).
So, we must check if there is already a CLAIM_DEF for a Schema-DID pair, and not allow to override it on ledger. We now do the same for Schema (that is we can not override Schema with the existent name-version-origin triple).

Tasks:
1) do not allow to override CLAIM_DEF on ledger for already existent schem-did pair;
2) update docs and diagrams so that it's clear it's not possible to create multiple CLAIM_DEFS for the same schema-did pair.