Trunk 4446: Support deriving auto_expire_date from duration and duration units for Drug Orders

Changes for ticket https://issues.openmrs.org/browse/TRUNK-4446
There is a difference in behaviour. When you leave out the column, it tries to set hl7_code as empty string and fails on unique hl7_code constraint (as emr api also adds a source with blank hl7_code).

Where as if you use `<column name="hl7_code"/>`, it inserts NULL value for hl7_code and does not violate the constraint.