optimized resource usage / streamlined DNS

1.4.0

some elected changelog items

(see https://github.com/deltachat/chatmail/blob/main/CHANGELOG.md for full list of changes)

• Add disable_ipv6 config option to chatmail.ini.
  Required if the server doesn't have IPv6 connectivity.
  (#312)

• allow current K9/Thunderbird-mail releases to send encrypted messages
  outside by accepting their localized "encrypted subject" strings.
  (#370)

• Migrate and remove sqlite database in favor of password/lastlogin tracking
  in a user's maildir.
  (#379)

• Require pyinfra V3 installed on the client side,
  run ./scripts/initenv.sh to upgrade locally.
  (#378)

• BREAKING: new required chatmail.ini value 'delete_inactive_users_after = 100'
  which removes users from database and mails after 100 days without any login.
  (#350)

• Refine DNS checking to distinguish between "required" and "recommended" settings
  (#372)

• Make DNS-checking faster and more interactive, run it fully during "cmdeploy run",
  also introducing a generic mechanism for rapid remote ssh-based python function execution.
  (#346)

• Remove sieve to enable hardlink deduplication in LMTP
  (#343)

• dovecot: enable gzip compression on disk
  (#341)

• DKIM-sign Content-Type and oversign all signed headers
  (#296)

• Add nonci_accounts metric
  (#347)

• Multiplex HTTPS, IMAP and SMTP on port 443
  (#357)