raft: add a batch of interaction-driven conf change tests

Verifiy the behavior in various v1 and v2 conf change operations.
This also includes various fixups, notably it adds protection
against transitioning in and out of new configs when this is not
permissible.

There are more threads to pull, but those are left for future commits.

raft/interaction_test.go

@@ -22,6 +22,9 @@ import (
 )
 
 func TestInteraction(t *testing.T) {
+	// NB: if this test fails, run `go test ./raft -rewrite` and inspect the
+	// diff. Only commit the changes if you understand what caused them and if
+	// they are desired.
 	datadriven.Walk(t, "testdata", func(t *testing.T, path string) {
 		env := rafttest.NewInteractionEnv(nil)
 		datadriven.RunTest(t, path, func(d *datadriven.TestData) string {