Add a receive buffer.

Add a receive buffer.

The receive buffer kicks in the moment we get a packet which is out of order.
We store the packet in the buffer, and keep track of up to 4 ranges of bytes
of received data. When retransmission fills the first gap, we send all the
buffered data (up to the second gap if applicable) to the application.

4 byte ranges seems to be a good value for up to moderate (20%) packet loss.
This algorithm greatly reduces the amount of useless packets being sent.
A future improvement is sending the SACK information in the ACK packets,
so the congestion window can be kept large while avoiding packets being
resent unnecessarily.