Enable Cassandra merge operator to be called with a single merge operand

Summary:
Updating Cassandra merge operator to make use of a single merge operand when needed. Single merge operand support has been introduced in #2721.
Closes https://github.com/facebook/rocksdb/pull/2753

Differential Revision: D5652867

Pulled By: sagar0

fbshipit-source-id: b9fbd3196d3ebd0b752626dbf9bec9aa53e3e26a

utilities/cassandra/merge_operator.h

@@ -26,6 +26,8 @@ public:
                                  Logger* logger) const override;
 
   virtual const char* Name() const override;
+
+  virtual bool AllowSingleOperand() const override { return true; }
 };
 } // namespace cassandra
 } // namespace rocksdb