Fix a regression in CombinerExecutor implementation introduced with t…

…he ability to return multiple post actions.

src/main/java/io/vertx/core/net/impl/pool/CombinerExecutor.java

@@ -56,15 +56,14 @@ public void submit(Action<S> action) {
           }
           final Task task = a.execute(state);
           if (task != null) {
+            Task last = task.last();
             if (head == null) {
               assert tail == null;
-              tail = task;
-              for (Task next = tail.next();next != null;next = tail.next()) {
-                tail = tail.next();
-              }
+              tail = last;
               head = task;
             } else {
-              tail = tail.next(task);
+              tail.next(task);
+              tail = last;
             }
           }
         }

src/main/java/io/vertx/core/net/impl/pool/Task.java

@@ -20,13 +20,21 @@ public Task replaceNext(Task next) {
     return oldNext;
   }
 
+  public Task last() {
+    Task current = this;
+    Task next;
+    while ((next = current.next) != null) {
+      current = next;
+    }
+    return current;
+  }
+
   public Task next() {
     return next;
   }
 
-  public Task next(Task next) {
+  public void next(Task next) {
     this.next = next;
-    return next;
   }
 
   protected final void runNextTasks() {