channeldb+routing: move control tower interface to routing

This commit creates an empty shall for control tower in the routing
package. It is a preparation for adding event notification.

routing/mock_test.go

@@ -182,7 +182,7 @@ type mockControlTower struct {
 	sync.Mutex
 }
 
-var _ channeldb.ControlTower = (*mockControlTower)(nil)
+var _ ControlTower = (*mockControlTower)(nil)
 
 func makeMockControlTower() *mockControlTower {
 	return &mockControlTower{