Tests for TcpClient <--> TcpServer interactions

Hi everyone! :)

Here are some spock tests with data-driven structure. I wrote them to help to fix #248.

If you run them, there can be seen that TcpClient +TcpServer interaction cannot sustain number of messages other than 2. I mean, this Spec is now failing.

So, I assume, with #248 fixed, these tests should check that it is really fixed :)

Thanks :-D