submission: EHLO command: Remove obsolete pending_ehlo handling. This was used to wait with sending an EHLO reply until the proxy connection to the relay server completes. The previous commit made this functionality obsolete, since command input handling is blocked entirely until the proxy connection is ready.

submission: Restructure handling of QUIT command. - Avoid explicitly proxying QUIT command when the proxy connection is not ready: in that case the SMTP client connection will just send QUIT if appropriate, without waiting for reply. - Add timeout for proxied QUIT command, so that there are no problems when the relay server hangs after QUIT (addresses FIXME).

submission: Dropped unused field from struct client.

Implemented SMTP submission proxy service.