SMTP interaction theft via MITM

Summary

@duesee found it was possible for an active MITM to inject a plaintext collaborator ID and use that to steal collaborator SMTP interactions We patched this in the following release: https://portswigger.net/burp/releases/professional-community-2020-9-2

This issue is closely related to CVE-2011-0411, and due to our non-standard SMTP implementation, some vulnerability scanners incorrectly flag the patched server as being vulnerable.