message: Add `Distribution::Constant`.
diff: @@ -457,6 +457,7 @@ impl<'a, C: Context, DS: Strategy<DeliverySchedule>> MutableHandle<'a, C, DS> {
  enum Distribution {
  Uniform,
  + Constant,
  Poisson(f64),
  }
  @@ -511,7 +512,7 @@ impl<C: Context<ValidatorId = ValidatorId>, DS: Strategy<DeliverySchedule>>
  delivery_strategy: None,
  weight_limits: (0, 0),
  start_time: Timestamp::zero(),
  - weight_distribution: Distribution::Uniform,
  + weight_distribution: Distribution::Constant,
  instance_id,
  seed: 0,
  round_exp: 12,
  @@ -604,11 +605,12 @@ impl<C: Context<ValidatorId = ValidatorId>, DS: Strategy<DeliverySchedule>>
  let start_time = self.start_time;
  let weights: Vec<Weight> = match self.weight_distribution {
  - Distribution::Uniform => {
  + Distribution::Constant => {
  let (lower, upper) = self.weight_limits;
  let weight = Weight((lower + upper) / 2);
  (0..validators_num).map(|_| weight).collect()
  }
  + Distribution::Uniform => unimplemented!(),
  // https://casperlabs.atlassian.net/browse/HWY-116
  Distribution::Poisson(_) => unimplemented!("Poisson distribution of weights"),
  };
  