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
uuid() is not a standard SQL function there are example changesets that do it in a way specific to our supported DBMSs. @djazayeri do we want to make this a core concept source? If yes, then this should be a pre-generated uuid