Export support for mixed update/insert against db2

The DB2 connector need to implemented the option --update-mode allowinsert.
option --update-mode allowinsert which can be used when loading into a table that is not empty.
In allowinsert mode export will try to update the row but if ithe row does not exist it will insert it. Without this option only rows that already exist in the table will be updated and any new rows are ignored.