RDT 1.0

implementation of rdt 1.0.
1. client sends an initial data packet, with message contents <filename>.
2. server converts <filename> into a series of packets.
3. server sends data packets one at a time, waits for ACK from client, repeat.
4. server sends FIN when there's no more data packets to send.
5. client shuts down when it gets a FIN.