smtp: improve pipelining support

[catenacyber]

Fixes #1863

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Link to redmine ticket:
https://redmine.openinfosecfoundation.org/issues/1863

Describe changes: Improve SMTP pipelining support

• Checks if the server includes pipelining support in its first multi-line reply
• Then , enter data mode immediately after the DATA command, without waiting for the server confirmation

PRScript output (if applicable): not applicable

[victorjulien]

Thanks Philippe! Could you also submit a test to https://github.com/OISF/suricata-verify ?

[catenacyber]

I modified SMTPParserTest03 in src/app-layer-smtp.c (whose comment was "Testing parsing pipelined commands")
Is that ok ?

[victorjulien]

If you can find (or create) a pcap, I would prefer to add that as well.

[catenacyber]

Ok, see OISF/suricata-verify#5

[victorjulien]

Merged as part of #3590, thanks Philippe!