Method LockGuard

src/common/lock_manager.h:86–88

 public:
  template <typename KeyType>
  explicit LockGuard(LockManager *lock_mgr, const KeyType &key) : lock_(lock_mgr->Get(key)) {
    lock_->lock();
  }
  ~LockGuard() {
    if (lock_) lock_->unlock();
  }