Move validatePromise(...) to ChannelHandlerInvokerUtil. Related to [#2398]

Motivation:
Once a user implement a custom ChannelHandlerInvoker it is needed to validate the ChannelPromise. We should expose a utility method for this.

Modifications:
Move validatePromise(...) from DefaultChannelHandlerInvoker to ChannelHandlerInvokerUtil and make it public.

Result:
User is able to reuse code

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

@@ -16,6 +16,8 @@
 
 package io.netty.channel;
 
+import io.netty.util.internal.StringUtil;
+
 import java.net.SocketAddress;
 
 import static io.netty.channel.DefaultChannelPipeline.*;
@@ -174,6 +176,44 @@ public final class ChannelHandlerInvokerUtil {
         invokeFlushNow(ctx);
     }
 
+    public static boolean validatePromise(
+            ChannelHandlerContext ctx, ChannelPromise promise, boolean allowVoidPromise) {
+        if (ctx == null) {
+            throw new NullPointerException("ctx");
+        }
+
+        if (promise == null) {
+            throw new NullPointerException("promise");
+        }
+
+        if (promise.isDone()) {
+            if (promise.isCancelled()) {
+                return false;
+            }
+            throw new IllegalArgumentException("promise already done: " + promise);
+        }
+
+        if (promise.channel() != ctx.channel()) {
+            throw new IllegalArgumentException(String.format(
+                    "promise.channel does not match: %s (expected: %s)", promise.channel(), ctx.channel()));
+        }
+
+        if (promise.getClass() == DefaultChannelPromise.class) {
+            return true;
+        }
+
+        if (!allowVoidPromise && promise instanceof VoidChannelPromise) {
+            throw new IllegalArgumentException(
+                    StringUtil.simpleClassName(VoidChannelPromise.class) + " not allowed for this operation");
+        }
+
+        if (promise instanceof AbstractChannel.CloseFuture) {
+            throw new IllegalArgumentException(
+                    StringUtil.simpleClassName(AbstractChannel.CloseFuture.class) + " not allowed in a pipeline");
+        }
+        return true;
+    }
+
     private static void notifyHandlerException(ChannelHandlerContext ctx, Throwable cause) {
         if (inExceptionCaught(cause)) {
             if (logger.isWarnEnabled()) {

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

@@ -20,7 +20,6 @@ import io.netty.util.Recycler;
 import io.netty.util.ReferenceCountUtil;
 import io.netty.util.concurrent.EventExecutor;
 import io.netty.util.internal.OneTimeTask;
-import io.netty.util.internal.StringUtil;
 
 import java.net.SocketAddress;
 
@@ -397,44 +396,6 @@ public class DefaultChannelHandlerInvoker implements ChannelHandlerInvoker {
         invokeWrite(ctx, msg, true, promise);
     }
 
-    private static boolean validatePromise(
-            ChannelHandlerContext ctx, ChannelPromise promise, boolean allowVoidPromise) {
-        if (ctx == null) {
-            throw new NullPointerException("ctx");
-        }
-
-        if (promise == null) {
-            throw new NullPointerException("promise");
-        }
-
-        if (promise.isDone()) {
-            if (promise.isCancelled()) {
-                return false;
-            }
-            throw new IllegalArgumentException("promise already done: " + promise);
-        }
-
-        if (promise.channel() != ctx.channel()) {
-            throw new IllegalArgumentException(String.format(
-                    "promise.channel does not match: %s (expected: %s)", promise.channel(), ctx.channel()));
-        }
-
-        if (promise.getClass() == DefaultChannelPromise.class) {
-            return true;
-        }
-
-        if (!allowVoidPromise && promise instanceof VoidChannelPromise) {
-            throw new IllegalArgumentException(
-                    StringUtil.simpleClassName(VoidChannelPromise.class) + " not allowed for this operation");
-        }
-
-        if (promise instanceof AbstractChannel.CloseFuture) {
-            throw new IllegalArgumentException(
-                    StringUtil.simpleClassName(AbstractChannel.CloseFuture.class) + " not allowed in a pipeline");
-        }
-        return true;
-    }
-
     private void safeExecuteInbound(Runnable task, Object msg) {
         boolean success = false;
         try {