[HttpCache] remove validation related headers when needed

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