[HttpKernel] made the classmap lazy-loaded

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| License       | MIT

see https://github.com/symfony/symfony-standard/pull/437#issuecomment-14667140

Instead of managing this from HttpCache, I suggest to handle it at Kernel's level, what do you think?