[Performance Branch] HashLinkList to avoid to convert length prefixed string back to internal keys

Summary: Converting from length prefixed buffer back to internal key costs some CPU but it is not necessary. In this patch, internal keys are pass though the functions so that we don't need to convert back to it.

Test Plan: make all check

Reviewers: haobo, kailiu

Reviewed By: kailiu

CC: igor, leveldb

Differential Revision: https://reviews.facebook.net/D15393

include/rocksdb/memtablerep.h

@@ -51,7 +51,11 @@ class MemTableRep {
    public:
     // Compare a and b. Return a negative value if a is less than b, 0 if they
     // are equal, and a positive value if a is greater than b
-    virtual int operator()(const char* a, const char* b) const = 0;
+    virtual int operator()(const char* prefix_len_key1,
+                           const char* prefix_len_key2) const = 0;
+
+    virtual int operator()(const char* prefix_len_key,
+                           const Slice& key) const = 0;
 
     virtual ~KeyComparator() { }
   };