Refactor DAO code

Currently the DAO code is not conforming to JDBC programming best practices.

There is a lot of duplicated code, especially all code handling statement and result set closing that is not automatically called and must be added to every business method by hand.

Following issue NAS-1816, the DAO code will contain a lot of try {} finally

code which is equally bad, and error prone.

Hence we should move from this very basic DAO implementation to some DAO framework that offers a higher level of abstraction.

Something like Spring JdbcTemplate seems to be the level of abstraction we need.