Write a file `uuid` in server's data directory that contains random UUID, generated at startup if file does not exist.

It will be used for several purposes:
- to anonymously identify server in Sentry (currently Sentry is unusable);
- (possible future usage) to identify server in ensemble.

@tavplubix comments?
{
    const int sleep_max_ms = 1000 * seconds_to_wait;
    const int sleep_one_ms = 100;
    int sleep_current_ms = 0;
    int current_connections = 0;
    for (;;)
    {
        current_connections = 0;
        for (auto & server : servers)
        {
            server.stop();
            current_connections += server.currentConnections();
        }
        if (!current_connections)
            break;
        sleep_current_ms += sleep_one_ms;
        if (sleep_current_ms < sleep_max_ms)
            std::this_thread::sleep_for(std::chrono::milliseconds(sleep_one_ms));
        else
            break;
    }
    return current_connections;
}
Poco::Net::SocketAddress makeSocketAddress(const std::string & host, UInt16 port, Poco::Logger * log)