Add a delay between two certifications issued by a same member
Currently, the protocol does not allow more than 1 certification by member and by block (excepted root block). To avoid massive Sybil attack, it would be good to strenghen this rule by adding a delay between two certifications of the same member.
For example, each certification might require to see a delay of 1 day before validating another one.
The idea comes from #65 (closed).