This is first cset for WL#1394 "Optimize index merge when all involved index ranges include only values with equal keys"

The main idea is to exploit the fact that key scans for "key=const" return ordered sequences of rowids.

sql/handler.h

@@ -382,6 +382,11 @@ public:
      false otherwise
   */
   virtual bool primary_key_is_clustered() { return false; }
+
+  virtual int cmp_ref(const byte *ref1, const byte *ref2)
+  {
+    return memcmp(ref1, ref2, ref_length);
+  }
 };
 
 	/* Some extern variables used with handlers */