reset greeting when upgrade to tls

Ref: 5.2 Result of `STARTTLS` command [rfc2487](https://www.ietf.org/rfc/rfc2487.txt)

> The server MUST discard any knowledge
   obtained from the client, such as the argument to the EHLO command,
   which was not obtained from the TLS negotiation itself.

> The client SHOULD send an EHLO command as the
   first command after a successful TLS negotiation.