[DS-1738] Support NUMERIC columns in database query result sets.

NUMERIC and DECIMAL results are now converted to BigDecimal, not
Integer resp. Long.
My mistake! That's what I get for skimming code from the GitHub UI ;)

I think your basic sanity test in testGetNumericColumn actually seems "good enough".

As for refactoring some of the DB logic, it seems reasonable as long as our Unit Tests don't complain.