Enhancing accumulator to allow subclasses to indicate whether they are ready for the batch to be processed. Default behaviour returns true.

Change-Id: I53a3ffc3ecd75ed2607f155a61971e05a6009a66
diff --git a/utils/misc/src/main/java/org/onlab/util/Accumulator.java b/utils/misc/src/main/java/org/onlab/util/Accumulator.java
index 568e38c..20b7a48 100644
--- a/utils/misc/src/main/java/org/onlab/util/Accumulator.java
+++ b/utils/misc/src/main/java/org/onlab/util/Accumulator.java
@@ -40,6 +40,10 @@
      */
     void processItems(List<T> items);
 
-    //TODO consider a blocking version that required consumer participation
-
+    /**
+     * Indicates whether the accumulator is ready to process items.
+     *
+     * @return true if ready to process
+     */
+    boolean isReady();
 }