Reduce IdleChannel allocations, close #1462

Motivation:

We allocate an AtomicBoolean for every IdleChannel.
We can avoid this cost, all the more as we allocate a IdleChannel just
to be able to remove it from the partition.

Modification:

Use an AtomicIntegerFieldUpdater instead.

Result:

Less allocations caused by IdleChannel.

client/src/main/java/org/asynchttpclient/netty/channel/DefaultChannelPool.java

@@ -15,18 +15,14 @@
 
 import static org.asynchttpclient.util.Assertions.assertNotNull;
 import static org.asynchttpclient.util.DateUtils.unpreciseMillisTime;
-import io.netty.channel.Channel;
-import io.netty.channel.ChannelId;
-import io.netty.util.Timeout;
-import io.netty.util.Timer;
-import io.netty.util.TimerTask;
 
 import java.net.InetSocketAddress;
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentLinkedDeque;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
 import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.stream.Collectors;
@@ -36,6 +32,12 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelId;
+import io.netty.util.Timeout;
+import io.netty.util.Timer;
+import io.netty.util.TimerTask;
+
 /**
  * A simple implementation of {@link ChannelPool} based on a {@link java.util.concurrent.ConcurrentHashMap}
  */
@@ -106,17 +108,21 @@ private static final class ChannelCreation {
     }
 
     private static final class IdleChannel {
+
+        private static final AtomicIntegerFieldUpdater<IdleChannel> ownedField = AtomicIntegerFieldUpdater.newUpdater(IdleChannel.class, "owned");
+
         final Channel channel;
         final long start;
-        final AtomicBoolean owned = new AtomicBoolean(false);
+        @SuppressWarnings("unused")
+        private volatile int owned = 0;
 
         IdleChannel(Channel channel, long start) {
             this.channel = assertNotNull(channel, "channel");
             this.start = start;
         }
 
         public boolean takeOwnership() {
-            return owned.compareAndSet(false, true);
+            return ownedField.getAndSet(this, 1) == 0;
         }
 
         public Channel getChannel() {