reduce ipip tunnel MTU by 20 bytes to accommodate the tunnel overhead

[murali-reddy]

Fixes #108

[murali-reddy]

With this fix results have improved drastically. IPIP tunnle overhead is minimal. Please see #102 for more results.

# iperf3 -c 100.96.1.4
Connecting to host 100.96.1.4, port 5201
[  4] local 100.96.2.3 port 52850 connected to 100.96.1.4 port 5201
[ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
[  4]   0.00-1.00   sec  97.5 MBytes   817 Mbits/sec  306    240 KBytes
[  4]   1.00-2.00   sec  87.5 MBytes   734 Mbits/sec    1    386 KBytes
[  4]   2.00-3.00   sec  95.0 MBytes   797 Mbits/sec    5    317 KBytes
[  4]   3.00-4.00   sec  96.2 MBytes   808 Mbits/sec    0    496 KBytes
[  4]   4.00-5.00   sec  96.2 MBytes   807 Mbits/sec    8    510 KBytes
[  4]   5.00-6.00   sec   101 MBytes   849 Mbits/sec    0    648 KBytes
[  4]   6.00-7.00   sec  97.5 MBytes   818 Mbits/sec    1    547 KBytes
[  4]   7.00-8.00   sec   100 MBytes   839 Mbits/sec    0    675 KBytes
[  4]   8.00-9.00   sec  96.2 MBytes   807 Mbits/sec    5    601 KBytes
[  4]   9.00-10.00  sec  97.5 MBytes   818 Mbits/sec    0    716 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-10.00  sec   965 MBytes   809 Mbits/sec  326             sender
[  4]   0.00-10.00  sec   957 MBytes   802 Mbits/sec                  receiver

iperf Done.