By increasing the size of the intermediate buffer from 8k to 16k,
perfomance of raw TCP forwarding is increased by ~19.4%.

Performance was measured using `iperf` on local network.

Direct:

> 0.0000-10.0091 sec   120 GBytes   103 Gbits/sec

Forwarding using `socat`:

> 0.0000-10.0155 sec  39.5 GBytes  33.9 Gbits/sec

Forwarding using `jetsocat` without this patch:

> 0.0000-10.0016 sec  52.1 GBytes  44.7 Gbits/sec

~31.8% faster than `socat`.

Forwarding using `jetsocat` with this patch:

> 0.0000-10.0076 sec  62.2 GBytes  53.4 Gbits/sec

~57.5% faster than `socat` and ~19.4% faster than without this patch.

Forwarding using `jetsocat`’s JMUX proxy (unrelated to this patch):

> 0.0000-10.0519 sec  8.27 GBytes  7.07 Gbits/sec

Using the JMUX proxy, even for a simple TCP port forwarding, is
currently a whole ~86.7 slower than regular TCP port forwarding.