code: public static Delay linear(TimeUnit unit, long upper, long lower) {
          return linear(unit, upper, lower, 1);
      }
docs: Creates a new {@link LinearDelay} with a custom boundaries and the default factor.
  
  @param unit the unit of the delay.
  @param upper the upper boundary.
  @param lower the lower boundary.
  @return a created {@link LinearDelay}.