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?
{
    Poco::Logger::root().information("Starting ClickHouse Keeper " + std::string{VERSION_STRING}
        + " with revision " + std::to_string(ClickHouseRevision::getVersionRevision())
        + ", " + build_id_info
        + ", PID " + std::to_string(getpid()));
}
}