/reviewed-on !611

This turnserver work on IPv6 but to allow communication with all devices, external-ip parameter is used to set public IPv4 which will be used by client.

/reviewed-on !610

Caddy and ATS shall have same timeouts for the requests.

It defaults to 600s, which is good reasonable chosen before.

ATS by default tries 4 times (1 non configurable, 3 retries) a given query to
the backend. Despite of "connection" in the name, ATS waits for first byte
from the backend, and it means that backend might already processing the
request, just no bytes are sent. In order to avoid this allow ATS to try only
once.

unlike what I assumed, env argument of ConfigParser.get does not work as
dict.get, it has priority.

This broke the case of SLAPOS-EGG-TEST

Rewrite this using ConfigParser.has_option

  The Remote User is managed on the backend apache which will unset it anyway.

As shown by https://github.com/zopefoundation/Zope/pull/655 and
https://github.com/Pylons/waitress/commit/6d4dab6bed88917b973066a6d5222917661802b7
backends usually don't accept headers with underscores.

SSL_CLIENT_SERIAL is removed because it's unused.

This fixes previous commit.

If backend does not send Content-Type the frontend is responsible of adding
correct one.