Add weak pointer handling (#3763)

test/actor/actor.test.cpp

@@ -80,7 +80,8 @@ TEST(Actor, DestructionBlocksOnReceive) {
 TEST(Actor, DestructionBlocksOnSend) {
     // Destruction blocks until the actor is not being sent a message.
 
-    struct TestScheduler : public Scheduler {
+    // NOTE: Any derived class must invalidate `weakFactory` in the destructor
+    struct TestScheduler final : public Scheduler {
         std::promise<void> promise;
         std::future<void> future;
         std::atomic<bool> waited;