TAJO-836

At first, I worried about the duplicated serialized name of different operators. As you know, gson does not allow the same serialized name of different objects. 
However, only some DDL operators such as CreateDatabase, CreateTable, Insert, and CreateIndex have the 'params' value, and these operators are never connected in a query plan. So,  it will be free from the gson restriction for the same name. 
I'll change it. Thanks!