Switch to scapy with python 3.

New scapy 2.4.0 buffers packets at the bfp layer. Call sniff() to
clear this list when starting the sniffer thread. The TCP sequence
number check in sr1() became more strict. Use the sniffer threa

New scapy 2.4.0 buffers packets at the bfp layer. Call sniff() to
clear this list when starting the sniffer thread. The TCP sequence
number check in sr1() became more strict. Use the sniffer thread
to capture retransmitted packetes which the new TCP answers() check
ignores now.

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Wait for the splicing syscall by grepping it in the relay log. This
ensures that scapy's SYN+ACK packet hits the TCP stack when it hurts.

Test more variations of TCP packets in the scapy splicing tests.
Delay connect and SYN+ACK to splice data into a connecting socket.