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

source/core/utils/value.cpp

@@ -84,6 +84,10 @@ bool value::is_time() const {
   return type_ == basic_type::Time;
 }
 
+bool value::is_timestamp() const {
+  return type_ == basic_type::DateTime;
+}
+
 bool value::is_blob() const {
   return type_ == basic_type::Blob;
 }