Proper feedback sequence numbers.

* Data length is part of DataHeader.
* Reconstruct data packet from payload.
* Ensure sizes are kept.
* int -> int16_t for block code.

* sendQueue is a pipe.
* receiveQueue is a B-tree.
* recv_sync mode yields packet that are due.

Conflicts:
	src/prrt/block.h
	src/prrt/packet.c
	src/prrt/processes/data_receiver.c
	src/prrt/processes/data_transmitter.c
	src/prrt/processes/feedback_receiver.c
	src/prrt/socket.c
	src/prrt/socket.h
	src/prrt/stores/forward_packet_table.c
	src/receiver.c
	src/sender.c
	src/util/list.c
	src/util/list.h