message: make durability helper panic on an undefined key
diff: @@ -93,9 +93,10 @@ where
  }
  fn durability(&self, _db: &DB, key: &Q::Key) -> Durability {
  - self.slot(key)
  - .map(|slot| slot.stamped_value.read().durability)
  - .unwrap_or(Durability::LOW)
  + match self.slot(key) {
  + Some(slot) => slot.stamped_value.read().durability,
  + None => panic!("no value set for {:?}({:?})", Q::default(), key),
  + }
  }
  fn entries<C>(&self, _db: &DB) -> C
  