Race condition with completion of Channel closeFuture and unregistration.

Motivation:
AbstractChannel.close() completes the closeFuture and then proceeds to
schedule a task to fireChannelInactive and unregister the channel. If
the user shuts down the EventLoop as a result of the closeFuture
completing this can result in a RejectedExecutionException when the
unregister task is scheduled.  This is mostly noise, but it does
somewhat pollute the logs.

Modifications:
Changed AbstractChannel.close() to not complete the channelFuture and
promise until after the unregistration task is scheduled.

Result:
Noisy error log should no longer be an issue.

transport/src/main/java/io/netty/channel/AbstractChannel.java

@@ -617,13 +617,11 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
             ChannelOutboundBuffer outboundBuffer = this.outboundBuffer;
             this.outboundBuffer = null; // Disallow adding any messages and flushes to outboundBuffer.
 
+            Throwable error = null;
             try {
                 doClose();
-                closeFuture.setClosed();
-                safeSetSuccess(promise);
             } catch (Throwable t) {
-                closeFuture.setClosed();
+                error = t;
-                safeSetFailure(promise, t);
             }
 
             // Fail all the queued messages
@@ -647,6 +645,14 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
                         }
                     });
                 }
+
+                // Now complete the closeFuture and promise.
+                closeFuture.setClosed();
+                if (error != null) {
+                    safeSetFailure(promise, error);
+                } else {
+                    safeSetSuccess(promise);
+                }
             }
         }