[Enhancement] local-tun TCP stack performance

[zonyitoo]

discussion moved from #745 .

iperf3 test in local network:

Connecting to host 192.168.1.110, port 5201
[  4] local 10.255.0.1 port 58580 connected to 192.168.1.110 port 5201
[ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
[  4]   0.00-1.00   sec  82.1 MBytes   689 Mbits/sec    0   87.0 KBytes
[  4]   1.00-2.00   sec  73.9 MBytes   620 Mbits/sec    0   87.0 KBytes
[  4]   2.00-3.00   sec  81.1 MBytes   680 Mbits/sec    0   87.0 KBytes
[  4]   3.00-4.00   sec  79.1 MBytes   664 Mbits/sec    0   87.0 KBytes
[  4]   4.00-5.00   sec  82.5 MBytes   692 Mbits/sec    0   87.0 KBytes
[  4]   5.00-6.00   sec  78.3 MBytes   657 Mbits/sec    0   87.0 KBytes
[  4]   6.00-7.00   sec  75.1 MBytes   630 Mbits/sec    0   87.0 KBytes
[  4]   7.00-8.00   sec  70.6 MBytes   592 Mbits/sec    0   87.0 KBytes
[  4]   8.00-9.00   sec  65.3 MBytes   547 Mbits/sec    0   91.2 KBytes
[  4]   9.00-10.00  sec  77.0 MBytes   647 Mbits/sec    0   91.2 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-10.00  sec   765 MBytes   642 Mbits/sec    0             sender
[  4]   0.00-10.00  sec   764 MBytes   641 Mbits/sec                  receiver

iperf Done.

The local network should be capable to handle 1000M. Pure tunnel shows result:

Connecting to host 127.0.0.1, port 5201
[  4] local 127.0.0.1 port 51474 connected to 127.0.0.1 port 5201
[ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
[  4]   0.00-1.00   sec   109 MBytes   918 Mbits/sec    0   3.00 MBytes
[  4]   1.00-2.00   sec  95.9 MBytes   804 Mbits/sec    0   3.00 MBytes
[  4]   2.00-3.00   sec   101 MBytes   849 Mbits/sec    0   3.00 MBytes
[  4]   3.00-4.00   sec  96.7 MBytes   811 Mbits/sec    0   3.00 MBytes
[  4]   4.00-5.00   sec   102 MBytes   860 Mbits/sec    0   3.00 MBytes
[  4]   5.00-6.00   sec   103 MBytes   864 Mbits/sec    0   3.00 MBytes
[  4]   6.00-7.00   sec   102 MBytes   853 Mbits/sec    0   3.00 MBytes
[  4]   7.00-8.00   sec   102 MBytes   853 Mbits/sec    0   3.00 MBytes
[  4]   8.00-9.00   sec  98.4 MBytes   826 Mbits/sec    0   3.00 MBytes
[  4]   9.00-10.00  sec  96.2 MBytes   808 Mbits/sec    0   3.00 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-10.00  sec  1007 MBytes   844 Mbits/sec    0             sender
[  4]   0.00-10.00  sec   997 MBytes   836 Mbits/sec                  receiver

iperf Done.