Prevent duplicate event when anchoring reg or cred in multisigs

[rodolfomiranda]

This PR fixes an inconsistency that occurs when a member of a multisig joins an anchoring event (for a registry creation or credential issuance/revocation) and when it's KEL already contains that event, resulting in a new event with the same anchor. That situation can be easily replicated if the threshold of the multisig is less that the total number of members, and the last member joins with a time delay that allows the propagation of the event.

To fix that problem, the code now checks if the last event already has the anchor event and avoid creating a new event in the KEL.