[ProxyManager] performance optimization for lazy loaded services

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes (well more a performance enhancement
| BC breaks?    | no
| Deprecations? | -
| Tests pass?   | -
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Result of a discussion at DrupalCamp Vienna.
The idea is essentially to no longer have to use a proxy once the service has been loaded.

Given a lazy loaded service ``foo`` with a method ``foo()``:
```
// proxy
var_dump(get_class($this->container->get('foo')));
// still proxy
var_dump(get_class($this->container->get('foo')));
$this->container->get('foo')->foo();
// no longer a proxy
var_dump(get_class($this->container->get('foo')));
```

Haven't looked at the tests yet, wanted to get feedback if this makes sense first.

/cc @dawehner @ocramius