Remove

```
[gcc:python2]
part = gcc-8.5
```

from neoppod/software-common.cfg and dream/software.cfg as it causes
gcc being pinned to a different version before and after rebootstrap
when the initial python version is not Python2.

This reverts dc91e5d4.

When use-ipv6 is enabled, make mariadb publish an url of the form:
  `mysql://user:password@[ipv6]:port/database`

instead of:
  `mysql://user:password@ipv6:port/database`

which results in the zope instances crashing during processing
due to urlparse.urlsplit failing to parse the url.

See merge request nexedi/slapos!1714

Fixes handling failed backups and default_exception_handler.

See merge request !1685

Stop using deprecated ssl.wrap_socket, and let unset key-file mean
the key is included in cert-file, as is done in python ssl module.
Also let the server crash if specified cert-file or key-file does
not exist, instead of silently falling back to non-TLS http.

Add 'socketpath' and 'abstract' options for path based unix sockets
and abstract unix sockets respectively.

See merge request !1716

See neoppod@f4a5fe3b.

This was sometimes causing false positives, for example if site have
only one hit and this hit takes more than one second, apdex is 0% and
promise failed, it seems useful to at least have a way to disable.

We are discussed on !1716 (comment 225941)
an idea of introducing a "minimal number of hits" parameter to not
detect this false positive on sites that are not accessed enough to
calculate a meaningful apdex.

Needed for password authentication to websocket
A bug was discovered in previous version

See merge request nexedi/slapos!1717

used in theia or with security vulnerabilities (idna & Werkzeug)