control/controlclient: add Auto.updateRoutine

Instead of having updates replace the map polls, create
a third goroutine which is solely responsible for making
sure that control is aware of the latest client state.

This also makes it so that the streaming map polls are only
broken when there are auth changes, or the client is paused.

Updates tailscale/corp#5761

Signed-off-by: Maisem Ali <maisem@tailscale.com>

control/controlclient/direct_test.go

@@ -42,7 +42,10 @@ func TestNewDirect(t *testing.T) {
 		t.Errorf("c.serverURL got %v want %v", c.serverURL, opts.ServerURL)
 	}
 
-	if !hi.Equal(c.hostinfo) {
+	// hi is stored without its NetInfo field.
+	hiWithoutNi := *hi
+	hiWithoutNi.NetInfo = nil
+	if !hiWithoutNi.Equal(c.hostinfo) {
 		t.Errorf("c.hostinfo got %v want %v", c.hostinfo, hi)
 	}