[HttpCache] remove validation related headers when needed

| Q             | A
| ------------- | ---
| Bug fix?      | [yes]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | #6977
| License       | MIT

I've added an embedded response counter because the current check was wrong, it was checking ```count($this->ttls)``` which is wrong because it isn't updated for validateable responses, ```add()``` is now only applied on esi responses and ```update()``` takes the master one at the end.

I've also fixed #6977 by removing validation related headers when there is at least one embedded response.