StorageService API for creating AsyncDocumentTree primitive

Change-Id: Ib7c3f19beb7b26a5b69161cf972c3c64d0be94b3
diff --git a/core/api/src/main/java/org/onosproject/store/service/StorageService.java b/core/api/src/main/java/org/onosproject/store/service/StorageService.java
index 4edb438..2e2acfb 100644
--- a/core/api/src/main/java/org/onosproject/store/service/StorageService.java
+++ b/core/api/src/main/java/org/onosproject/store/service/StorageService.java
@@ -114,12 +114,21 @@
      * @param <E> work element type
      * @param name work queue name
      * @param serializer serializer
-     *
      * @return WorkQueue instance
      */
     <E> WorkQueue<E> getWorkQueue(String name, Serializer serializer);
 
     /**
+     * Returns an instance of {@code AsyncDocumentTree} with specified name.
+     *
+     * @param <V> tree node value type
+     * @param name document tree name
+     * @param serializer serializer
+     * @return AsyncDocumentTree instance
+     */
+    <V> AsyncDocumentTree<V> getDocumentTree(String name, Serializer serializer);
+
+    /**
      * Returns an instance of {@code Topic} with specified name.
      *
      * @param <T> topic message type