Addressing comments

Author: shorea

http-clients/netty-nio-client/src/main/java/software/amazon/awssdk/http/nio/netty/internal/ReleaseOnceChannelPool.java

@@ -80,6 +80,8 @@ public Future<Void> release(Channel channel, Promise<Void> promise) {
     }
 
     private boolean shouldRelease(Channel channel) {
+        // IS_RELEASED may be null if this channel was not acquired by this pool. This can happen
+        // for HTTP/2 when we release the parent socket channel
         return channel.attr(IS_RELEASED).get() == null
                || channel.attr(IS_RELEASED).get().compareAndSet(false, true);
     }

http-clients/netty-nio-client/src/main/java/software/amazon/awssdk/http/nio/netty/internal/ResponseHandler.java

@@ -215,8 +215,6 @@ private void onCancel() {
                     } finally {
                         runAndLogError("Could not release channel back to the pool",
                             () -> closeAndRelease(channelContext));
-                        runAndLogError("Could not release channel back to the pool",
-                            () -> closeAndRelease(channelContext));
                     }
                 }