message: Implement `register_signature` and `register_block`
diff: @@ -47,11 +47,19 @@ impl AccumulatedBlock {
  }
  fn register_signature(&mut self, finality_signature: FinalitySignature) {
  - todo!();
  + // TODO: What to do when we receive multiple valid finality_signature from single public_key?
  + // TODO: What to do when we receive too many finality_signature from single peer?
  + self.signatures
  + .insert(finality_signature.public_key.clone(), finality_signature);
  }
  fn register_block(&mut self, block: Block) {
  - todo!();
  + if self.block.is_some() {
  + warn!(block_hash = %block.hash(), "received duplicate block");
  + return;
  + }
  +
  + self.block = Some(block);
  }
  fn has_sufficient_signatures(&self) -> bool {
  