Add rolling update tests for Crdt gossip

[mvines]

#690 fixed a bug whereby new nodes coming online during a rolling update where receiving stale Crdt data from old nodes, "infecting" the new nodes with the stale data.

We could build up a testsuite that exercises various kinds of node updates a network such as leaders and validators coming and going and validating the Crdt remains sane. We should also throw invalid Crdt data at a simulated network with a large number of nodes too and validate it is ignored or purged.

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[stale]

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