Fix possible deadlock in DefaultChannelPipeline.destroyDown(...)

Motivation:

We need to ensure we not hold a lock while executor callHandlerRemoved(...) as this may lead to a deadlock if handlerRemoved(...) will call another method in DEfaultChannelPipeline from another thread that will need to obtain the lock as well and wait for the result.

Modifications:

Release the lock before call handlerRemoved0(...).

Result:

No more deadlock possible

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

@@ -858,8 +858,8 @@ public class DefaultChannelPipeline implements ChannelPipeline {
             if (inEventLoop || executor.inEventLoop(currentThread)) {
                 synchronized (this) {
                     remove0(ctx);
-                    callHandlerRemoved0(ctx);
                 }
+                callHandlerRemoved0(ctx);
             } else {
                 final AbstractChannelHandlerContext finalCtx = ctx;
                 executor.execute(new OneTimeTask() {