Add ValidatorInfo to PeerContacts.

[nibhar]

This PR adds Validators information to the PeerContacts exchanged during discovery. In this PR the information is not populated, neither is it utilized. It is intended as an additional means of retrieving validator information aside from just the dht.

In addition to that it changes how peer contact information gets discarded in case it is faulty. Not discarding everything, but only the faulty piece of data instead. As Validator contacts can only be verified when the state is complete, and only if the respective key has not been rotated out (or in) yet, an entry in the address book can be flagged as local_only in those cases where verification is not (yet) possible. They should be re-evaluated at a time, when the verification is possible. Local_only entries will not be put into discovery messages.