Add comment to the buffer_state_bit_.

  // for logging and debugging purposes only and should not be used for lookup

  // or any other functional purpose as a security precaution.

  int id_;

  // Channel id.

  int cid_;

  // Client bit mask which indicates the locations of this client object in the

  // buffer_state_.

  uint64_t buffer_state_bit_{0ULL};

  IonBuffer buffer_;

  IonBuffer metadata_buffer_;