Implement contains for SelectedSelectionKeySet (#13452)

Motivation:
The `contains` method may be called in the
`sub.nio.ch.SelectorImpl.processReadyEvents` method on macOS and
Windows, so we need to implement it correctly. Otherwise, we can lose
read-poll events.
See the investigation on the Hazelcast repo:
hazelcast/hazelcast#24267

Modification:
Add a contains method that just does a linear scan for the selection
key. This preserves the performance characteristics of all the other
methods.

Result:
Fixes #13328

transport/src/main/java/io/netty/channel/nio/SelectedSelectionKeySet.java

@@ -20,6 +20,7 @@
 import java.util.Arrays;
 import java.util.Iterator;
 import java.util.NoSuchElementException;
+import java.util.Objects;
 
 final class SelectedSelectionKeySet extends AbstractSet<SelectionKey> {
 
@@ -51,6 +52,13 @@ public boolean remove(Object o) {
 
     @Override
     public boolean contains(Object o) {
+        SelectionKey[] array = keys;
+        for (int i = 0, s = size; i < s; i++) {
+            SelectionKey k = array[i];
+            if (k.equals(o)) {
+                return true;
+            }
+        }
         return false;
     }
 

transport/src/test/java/io/netty/channel/nio/SelectedSelectionKeySetTest.java

@@ -102,8 +102,8 @@ public void contains() {
         SelectedSelectionKeySet set = new SelectedSelectionKeySet();
         assertTrue(set.add(mockKey));
         assertTrue(set.add(mockKey2));
-        assertFalse(set.contains(mockKey));
-        assertFalse(set.contains(mockKey2));
+        assertTrue(set.contains(mockKey));
+        assertTrue(set.contains(mockKey2));
         assertFalse(set.contains(mockKey3));
     }