datastore: reduce DAO abstraction to remove complexity

StatementList and StatementElement were an attempt at creating our own
ORM for JAMS. However, doing an database query was very verbose on the
caller side, and if the 1st, 2nd or 4th field of StatementElement was
user input, we would be vulnerable to SQL injections. This patch
hardcoded the SQL queries in the DAO, making it easier on SQL expert to
audit the queries.

Change-Id: I4699bf3146a077efe03efd178b6190dfe60c72e9