forward iterator

Summary:
Forward iterator puts everything together in a flat structure instead of
a hierarchy of nested iterators. this should simplify the code and
provide better performance. It also enables more optimization since all
information are accessiable in one place.
Init evaluation shows about 6% improvement

Test Plan: db_test and db_bench

Reviewers: dhruba, igor, tnovak, sdong, haobo

Reviewed By: haobo

Subscribers: sdong, leveldb

Differential Revision: https://reviews.facebook.net/D18795

table/iterator_wrapper.h

@@ -8,6 +8,9 @@
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
 
 #pragma once
+
+#include "rocksdb/iterator.h"
+
 namespace rocksdb {
 
 // A internal wrapper class with an interface similar to Iterator that