[FEATURE] Reverse Proxy TTL handling

Currently the template cache life time is used as HTTP cache control (`Cache-Control:max-age=3600, public`) which is interpreted by browsers and reverse-proxies. To get more control over the reverse-proxy cache we introduced a new HTTP Header `X-Reverse-Proxy-TTL`. Separate options for HTTP cache control should be added (https://github.com/sulu-cmf/sulu-standard/issues/272)

This implementation enables the Symfony HttpCache to also validate the `X-Reverse-Proxy-TTL` setting.

Inspiration: https://github.com/liip/LiipCacheControlBundle#custom-varnish-time-outs

__Related PR's:__

https://github.com/sulu-cmf/sulu/pull/262
https://github.com/sulu-cmf/SuluWebsiteBundle/pull/59


__Tasks:__

- [ ] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no tests
| Fixed tickets | none
| BC Breaks     | none
| Doc           |