Request moving/transfering DOI transactions from fallback to responsible validator

(DRAFT!) As long as an email domain e.g. @example.com didn't register public key of Doichain node inside a DNS TXT value like so:

• doichain-opt-in-key=<doichain_publickey> or
• doichain-opt-in-provider=<doichain_publickey> all opt-ins will be validated through the public key of the so far centralized fall back server.

As soon as a email domain name registers it's Doichain node. We need a RPC call which allows transfering / rewriting DOI permissions onto for the new Doichain address.

1. Implement new RPC call "opt-ins/transfer" (parameter email domain, signature)
2. Request public key of email domain from DNS and generate the Doichain address for it
3. Get all DOI permission nameId's of a target-domain (this might be difficult, as we do not store recipients emails nor domains on the fallback server - need research / more implementation needed)
4. Get all transactions which holds the SOI permissions of this email domain and send it back to the new validator.
5. Make sure the validator is the right validator by checking the signature of the transferred email domain with the public key in the DNS of the email domain.