messageBoundary () {
    this._closeChunkIfOpen()

    if (this._buffer.remaining() < _CHUNK_HEADER_SIZE) {
      this.flush()
    }

    // Write message boundary
    this._buffer.writeInt16(_MESSAGE_BOUNDARY)
  }
Bolt messages are encoded in one or more chunks, and the boundary between two messages
is encoded as a 0-length chunk, `00 00`. This inserts such a message boundary, closing
any currently open chunk as needed