more-itertools is a hidden dependency, not managed by buildout.
It has dropped support for python2 after version 5.0.0

/cc @tomo @jm

/reviewed-on nexedi/slapos!536

This partition contains no data worthy of backup:
- only in-transit emails appear
- logging is (sadly) not local to the partition

- Use `xz-utils` component for log file compression
- Fix some stacks and software releases to use the logrotate stack instead of re-creating it:
  - `software/caddy-frontend`
  - `stack/resilient`

/reviewed-on nexedi/slapos!508

Use xz instead of gzip.
Also put the cron script in etc/service instead of etc/run (to have the
"cron-on-watch" process)

Building Bison 3.3.2 now requires automake.

OpenVPN fails to build on Debian testing because
net-tools is no longer provided by default.

/reviewed-on nexedi/slapos!534

in apache frontend, we have been using:

```
LogFormat "%h %l %{REMOTE_USER}i %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\" %D" combined
```

The %l is (from mod_log_config docs): Remote logname (from identd, if
supplied). This will return a dash unless mod_ident is present and
IdentityCheck is set On.

In the case of apache frontend, it was always a - . This is missing in
caddy frontend and our existing log processing tools (apachedex) cannot
be used on frontend logs since we switched to Caddy.

/reviewed-on nexedi/slapos!530

dict in headers is smallcase, so it was never working in reality.

Added assertion which proves that the ATS is serving stale content in case if
the backend does not work, according to RFC5861.

It is beleived that stale-while-revalidate will work the same way, but it is
much harder to test, thus it is not done directly.

Adapted configuration and instantiation to ATS 7.

Deployment:
 * traffic_line has been replaced with traffic_ctl
 * access log, of squid style, is ascii instead of binary, to do so
   logging.config is generated
 * ip_allow.config is configured to allow access from any host
 * RFC 5861 (stale content on error or revalidate) is implemented with core
   instead with deprecated plugin
 * trafficserver-autoconf-port renamed to trafficserver-synthetic-port
 * proxy.config.system.mmap_max removed, as it is not used by the system anymore

Tests:
 * As Via header is not returned to the client, it is dropped from the
   tests, instead its existence in the backend is checked.
 * Promise plugin trafficserver-cache-availability.py is re enabled, as
   it is expected to work immediately.

as suggested by Jerome on b26ae6b3

old URL doesn't exist anymore

Do not change any defaults regarding 1.9, 1.10 and 1.11.