code: public static Specification<JpaTarget> isOverdue(final long overdueTimestamp) {
          return (targetRoot, query, cb) -> cb.lessThanOrEqualTo(targetRoot.get(JpaTarget_.lastTargetQuery),
                  overdueTimestamp);
      }
docs: {@link Specification} for retrieving {@link Target}s that are overdue. A
  target is overdue if it did not respond during the configured
  intervals:<br>
  <em>poll_itvl + overdue_itvl</em>
  
  @param overdueTimestamp
  the calculated timestamp to compare with the last respond of a
  target (lastTargetQuery).<br>
  The <code>overdueTimestamp</code> has to be calculated with
  the following expression:<br>
  <em>overdueTimestamp = nowTimestamp - poll_itvl -
  overdue_itvl</em>
  
  @return the {@link Target} {@link Specification}