Add possibility of creating arbitrary queries on DBManager

[alcarraz]

Arbitrary query over the entity type (T) of this manager
Example usage:

GLTransactionManager mgr = new GLtransactionManager(db);
List<T> results = mgr.queryItems(true, (cb, root) ->
              cb.or(
                  cb.greaterThanOrEqualTo(root.get("postDate"), someDate),
                  cb.isNull(root.get("postDate")
              )
         );