Re-do sequential gossip in CAT

[evan-forbes]

we pieced together sequential gossip in CAT but adding such a feature normally would require a lot more thinking. the only reason it got through was due to time constraints and relying on testing CAT in production rather than properly with internal testnets.

we should write an ADR and refactor the code to enable sequential gossip

[ninabarbakadze]

We could even remove the sequential gossip in CAT because we notice that pull based gossip + sticky peers is sufficient