Abstract DB access

[dbonino]

As emerges from JDBCDPPPTDataServiceImpl.java currently a dabase-dependent approach is used which requires to develop and test specific queries for each available backend. Given the fact that the database structure is overly simple, I see no harms in exploiting the spring boot ORM (hibernate + hickary pool) to avoid hand carving of queries to handle variations of SQL dialects on different databases.