message: add refpack push_back_missing, push_front_missing
diff: @@ -33,7 +33,9 @@ pub struct RefPack(VecDeque<BlockHash>);
  impl RefPack {
  pub fn new() -> Self { RefPack(VecDeque::new()) }
  pub fn push_back(&mut self, bh: BlockHash) { self.0.push_back(bh) }
  + pub fn push_back_missing(&mut self) { self.0.push_back([0u8; HASH_SIZE]) }
  pub fn push_front(&mut self, bh: BlockHash) { self.0.push_front(bh) }
  + pub fn push_front_missing(&mut self) { self.0.push_front([0u8; HASH_SIZE]) }
  pub fn iter<'a>(&'a self) -> Iter<'a, BlockHash> { self.0.iter() }
  pub fn read<R: io::Read>(reader: &mut R) -> Result<Self> {
  