Alter cache max-age in response

I'm marking this as for review ... but I haven't even tested it myself yet...

Ugh ... okay - that's annoying. Seems this isn't possible without overriding the entire service as it contains;

\Drupal::service('page_cache_kill_switch')->trigger();

And it doesn't look like that's able to be reverted :( Lemme think about it, maybe there's another way ...

edit: I admit I'm maybe a niche case (long-term parking of domains) but this should still be possible IMO.

Ugh ... I'm just going to have to do this in Varnish instead. So much for Drupal being flexible.

pobster → closed merge request !2

Here's what I ended up doing:

sub vcl_recv {
  # Check if this is one of our sites in maintenance mode.
  if (
    req.http.host ~ "^(www\.)?drupal\.org$"
  ) {
    set req.http.X-Maintenance-Mode = "true";
  }
}

sub vcl_backend_response {
  # Check if we're in maintenance mode.
  if (bereq.http.X-Maintenance-Mode) {
    set beresp.http.Cache-Control = "public, max-age=86400";
    unset beresp.http.Set-Cookie;
    unset beresp.http.X-Maintenance-Mode;
    set beresp.ttl = 1d;
  }
}