avoiding creation of client sessions

[konrad-arena]

Currently when there is no _ga cookie then ClientIdProvider generates one. This makes browsers with ublock appear in analytics without any source/referrer and without any frontend pageviews and disrupt the statistics.

It would be great to have configuration option to make tracker ignore events when there is no cid

Currently ClientIdProvider::getClientId() is annotated with @return false|string but it seems that it is expected to return string and always returns string