state: switch consumers to NodeStore primary routes

Replace routes.PrimaryRoutes reads with NodeStore. Connect bumps
SessionEpoch; Disconnect re-checks it inside UpdateNode so the
check and mutation are atomic against a concurrent Connect on
the same node.

The connect_race regression test is carried in its final
SessionEpoch form.

Updates #3203

hscontrol/servertest/ha_dynamic_test.go

@@ -72,7 +72,7 @@ func TestHAFailover_ViewerSeesPrimaryFlip(t *testing.T) {
 			return hasPeerPrimaryRoute(nm, "dyn-flip-r1", route)
 		})
 
-	changed := srv.State().PrimaryRoutes().SetNodeHealthy(id1, false)
+	changed := srv.State().SetNodeUnhealthy(id1, true)
 	require.True(t, changed, "marking primary unhealthy should change primaries")
 
 	srv.App.Change(change.PolicyChange())