MseByteStream
A ByteStream that transparently RC4-encrypts writes and decrypts reads for an established MSE/PE connection. This is the live counterpart of libtorrent's rc4_handler sitting in bt_peer_connection's send/recv path. RC4 is a byte-stream cipher, so the chunk boundaries a PeerConnection happens to read/write in are irrelevant; only the total byte order matters, and the per-direction locks keep encrypt-order == wire-order.