Very poor obfsvpn KCP performance

Running tests in #37 (comment 1185171) it was found that obfsvpn + KCP was evidencing very poor performance:

❯ docker-compose exec client iperf3 -c 10.8.0.1 --bind-dev tun0
Connecting to host 10.8.0.1, port 5201
[  5] local 10.8.0.6 port 33516 connected to 10.8.0.1 port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  1.21 MBytes  10.2 Mbits/sec    3    239 KBytes       
[  5]   1.00-2.00   sec   632 KBytes  5.18 Mbits/sec    0    702 KBytes       
[  5]   2.00-3.00   sec   890 KBytes  7.29 Mbits/sec    0   1.05 MBytes       
[  5]   3.00-4.00   sec  0.00 Bytes  0.00 bits/sec    0   1.10 MBytes       
[  5]   4.00-5.00   sec  1.01 MBytes  8.44 Mbits/sec    0   1.14 MBytes       
[  5]   5.00-6.00   sec  0.00 Bytes  0.00 bits/sec    0   1.28 MBytes       
[  5]   6.00-7.00   sec  1.17 MBytes  9.81 Mbits/sec    0   1.46 MBytes       
[  5]   7.00-8.00   sec  0.00 Bytes  0.00 bits/sec    0   1.69 MBytes       
[  5]   8.00-9.00   sec  1.25 MBytes  10.5 Mbits/sec    0   1.97 MBytes       
[  5]   9.00-10.00  sec  0.00 Bytes  0.00 bits/sec    0   2.29 MBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  6.12 MBytes  5.14 Mbits/sec    3             sender
[  5]   0.00-13.11  sec  5.67 MBytes  3.63 Mbits/sec                  receiver

iperf Done.

There's nothing that immediately jumps out as to why this would be so bad, but it's obviously unacceptable performance so let's see what we can dig up...