Immediately close channel when subscribers cancel (#1459)

This commit includes two changes:
- More aggressively mark the ResponseFuture as done
  to avoid threading issues that still read the old
  state
- Directly invoke channel closing, since there is
  no guarantee of another channelRead occuring (e.g.
  if the stream was hard closed)

Author: HKrausAxon

Diff for: client/src/main/java/org/asynchttpclient/netty/handler/StreamedResponsePublisher.java

@@ -42,16 +42,15 @@ public StreamedResponsePublisher(EventExecutor executor, ChannelManager channelM
     protected void cancelled() {
         logger.debug("Subscriber cancelled, ignoring the rest of the body");
 
-        // The subscriber cancelled early, we need to drain the remaining elements from the stream
-        channelManager.drainChannelAndOffer(channel, future);
-        channel.pipeline().remove(StreamedResponsePublisher.class);
-
         try {
             future.done();
         } catch (Exception t) {
             // Never propagate exception once we know we are done.
             logger.debug(t.getMessage(), t);
         }
+
+        // The subscriber cancelled early - this channel is dead and should be closed.
+        channelManager.closeChannel(channel);
     }
 
     NettyResponseFuture<?> future() {