[fsmsock/base] TcpTransport: shutdown & close socket on disconnect

Signed-off-by: Anton D. Kachalov <mouse@yandex-team.ru>
[fsmsock/async] Register `atexit' handler to close all client's connections in a timely fashion way.

Signed-off-by: Anton D. Kachalov <mouse@yandex-team.ru>