Ability to simulate network problems

[grafin]

Currently there is no way to block traffic between nodes when using UNIX sockets. For some test current approach to disconnect instances via box.cfg.replication doesn't work, because it closes the connection and the other side notices the disconnect almost immediately (e.g. tarantool/tarantool#7141).

In two words, I want something like:

sudo iptables -I INPUT -p tcp --sport 3301 -j DROP
sudo iptables -I INPUT -p tcp --dport 3301 -j DROP

But for UNIX sockets.

It would be great, if there was a 'proxy' helper which could block all traffic, without closing the connection.

[grafin]

Done in faddd6c