We have added a new function, SystemCatalog::upgrade(), which will
create a new column in the database and fill it with default values.
It uses a reference column to fill the column, which is passed as a
std::unordered_map<key, value> value to SystemCatalog::upgrade()
function, where key is the target column which needs to be created.
In addition, we pass additional std::unordered_map's to
SystemCatalog::upgrade() containing information such as the datatypes
and colwidths of the target and reference columns, as well as the
default value which will be used to fill the target column.
