message: Lock only when needed in history
diff: @@ -780,8 +780,8 @@ namespace ccf
  bool verify_proof(const std::vector<uint8_t>& v) override
  {
  - std::lock_guard<std::mutex> guard(state_lock);
  Proof proof(v);
  + std::lock_guard<std::mutex> guard(state_lock);
  return replicated_state_tree.verify(proof);
  }
  @@ -794,9 +794,9 @@ namespace ccf
  void append(const std::vector<uint8_t>& data) override
  {
  - std::lock_guard<std::mutex> guard(state_lock);
  crypto::Sha256Hash rh({data.data(), data.size()});
  log_hash(rh, APPEND);
  + std::lock_guard<std::mutex> guard(state_lock);
  replicated_state_tree.append(rh);
  }
  