io: fix memory leak during shutdown (#3477)

In some cases, a cycle is created between I/O driver wakers and the I/O
driver resource slab. This patch clears stored wakers when an I/O
resource is dropped, breaking the cycle.

Fixes #3228

tests-integration/src/bin/test-mem.rs

@@ -0,0 +1,21 @@
+use futures::future::poll_fn;
+
+fn main() {
+    let rt = tokio::runtime::Builder::new_multi_thread()
+        .worker_threads(1)
+        .enable_io()
+        .build()
+        .unwrap();
+
+    rt.block_on(async {
+        let listener = tokio::net::TcpListener::bind("0.0.0.0:0").await.unwrap();
+        tokio::spawn(async move {
+            loop {
+                poll_fn(|cx| listener.poll_accept(cx)).await.unwrap();
+            }
+        });
+    });
+
+    std::thread::sleep(std::time::Duration::from_millis(50));
+    drop(rt);
+}