[alex3d]

This PR would greatly speedup bulk transfer:

• increase network buffer from 1kb to 16kb
• do not hexlify all transferred data in case the TRACE log level is disabled

Without these patches (1gb.dat is 1 GByte of random data):

$ python3 ./sshtunnel.py -U user -P password -L :8081 -R 127.0.0.1:80 -p 22 host
$ curl http://127.0.0.1:8081/1gb.dat -o /dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 1024M  100 1024M    0     0  10.0M      0  0:01:41  0:01:41 --:--:-- 10.2M

With increased network buffers:

$ curl http://127.0.0.1:8081/1gb.dat -o /dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 1024M  100 1024M    0     0  32.7M      0  0:00:31  0:00:31 --:--:-- 32.4M

With increased buffers and hexlify disabled:

$ curl http://127.0.0.1:8081/1gb.dat -o /dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 1024M  100 1024M    0     0  59.3M      0  0:00:17  0:00:17 --:--:-- 62.1M

[pahaz]

Welcome to the CONTRIBUTION list: aa7342e