added timestamp code, support columns for group by and order by and tested load has-many eager code

source/core/object/attribute.cpp

@@ -85,6 +85,10 @@ bool attribute::is_time() const {
   return type_ == utils::basic_type::Time;
 }
 
+bool attribute::is_timestamp() const {
+  return type_ == utils::basic_type::DateTime;
+}
+
 bool attribute::is_blob() const {
   return type_ == utils::basic_type::Blob;
 }