Fix to wait for final SMTP command before checking next one

[dragoangel]

1. Exporter connects to postscreen 220-postsreen ESMTP Postfix and response to it EHLO prober but it should wait final command 220 postfix ESMTP Postfix and only after it send EHLO prober.
2. We need to send STARTTLS only after getting last 250 .* capability from the list, as STARTTLS capability can be in any place, we need check we have it in the list and check that we get all capabilities at same time.

[dragoangel]

To note: after implementing this fix my ssl exporter stopped to throw any false positive responses that endpoint is down. For people who can't wait to get this merged I build own image: dragoangel/ssl-exporter:2.5.1

@ribbybibby please review