[RocksDB][Performance Branch] Make height and branching factor configurable for skiplist implementation

Summary: As title. Especially, HashSkipListRepFactory will be able to specify a relatively small height, to reduce the memory overhead of one skiplist per bucket.

Test Plan: make check and test it on leaf4

Reviewers: dhruba, sdong, kailiu

CC: reconnect.grayhat, leveldb

Differential Revision: https://reviews.facebook.net/D14307

include/rocksdb/memtablerep.h

@@ -267,9 +267,16 @@ public:
 
 // The same as TransformRepFactory except it doesn't use locks.
 // Experimental, will replace TransformRepFactory once we are sure
-// it performs better
+// it performs better. It contains a fixed array of buckets, each
+// pointing to a skiplist (null if the bucket is empty).
+// bucket_count: number of fixed array buckets
+// skiplist_height: the max height of the skiplist
+// skiplist_branching_factor: probabilistic size ratio between adjacent
+//                            link lists in the skiplist
 extern MemTableRepFactory* NewHashSkipListRepFactory(
-    const SliceTransform* transform, size_t bucket_count = 1000000);
+  const SliceTransform* transform, size_t bucket_count = 1000000,
+  int32_t skiplist_height = 4, int32_t skiplist_branching_factor = 4
+);
 
 }