Issues with two-node clusters

[cole-miller]

The MAAS and Juju teams have both run into trouble when operating two-node dqlite clusters. In both cases there's a desire for go-dqlite's automatic role management to support maintaining both nodes as voters in this situation; right now this is not possible due to the code here:

go-dqlite/app/roles.go

Lines 134 to 143 in 1940345

	// If the cluster is too small, make sure we have just one voter (us).
	if c.size() < minVoters {
	for node := range c.State {
	if node.ID == leader || node.Role != client.Voter {
	continue
	}
	return client.Spare, []client.NodeInfo{node}
	}
	return -1, nil
	}

You can't set the voter target to an even number. If you set it to 1 then the problem is obvious, and if you set it to 3 then this code still leads to having only one voter in a two-node cluster. We should fix this situation so that two-node clusters with two voters are viable with go-dqlite.