add seeforprev in history

Summary: update new feature in history and avoid breaking mongorocks

Test Plan: make check

Reviewers: sdong, yiwu, andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D64611

HISTORY.md

@@ -7,6 +7,9 @@
 ### New Features
 * Add DB::SetDBOptions() to dynamic change base_background_compactions and max_background_compactions.
 
+### New Features
+* Added Iterator::SeekForPrev(). This new API will seek to the last key that less than or equal to the target key.
+
 ## 4.12.0 (9/12/2016)
 ### Public API Change
 * CancelAllBackgroundWork() flushes all memtables for databases containing writes that have bypassed the WAL (writes issued with WriteOptions::disableWAL=true) before shutting down background threads.

include/rocksdb/iterator.h

@@ -72,7 +72,7 @@ class Iterator : public Cleanable {
   // Position at the last key in the source that at or before target
   // The iterator is Valid() after this call iff the source contains
   // an entry that comes at or before target.
-  virtual void SeekForPrev(const Slice& target) = 0;
+  virtual void SeekForPrev(const Slice& target) {}
 
   // Moves to the next entry in the source.  After this call, Valid() is
   // true iff the iterator was not positioned at the last entry in the source.