[DependencyInjection] Self-referenced 'service_container' service breaks garbage collection

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Needs merge to | 2.4
| Fixed tickets | —
| License       | MIT
| Doc PR        | —

#### Problem
1. `Container::__construct()` sets the `service_container` service ID to a self-reference of `$this`.
1. This breaks PHP garbage collection (as well as `clone`).

#### Background
* As explained in [Reference Counting Basics](http://php.net/manual/en/features.gc.refcounting-basics.php), PHP is not able to garbage collect variables that contain a reference to itself, because the `refcount` is always > 0.

#### Example Use-Case
A precompiled container that is cloned for multiple tests (because `::compile()` can be slow).

```php
$precompiled = new ContainerBuilder();
// ...
$precompiled->compile();
foreach ($precompiled->getServiceIds() as $id) {
  if ($precompiled->initialized($id)) {
    $precompiled->set($id, null);
  }
}
$precompiled_clean = clone $precompiled;
$precompiled = NULL;
// ...
foreach ($tests as $test) {
  $container = clone $precompiled_clean;
  // Required without this PR:
  $container->set('service_container', $container);
  // ...
}
```

* **Actual result [master]:**
  All references to the original `$precompiled` container are not destructed and cleaned up, because `$precompiled` still references itself, which can cause terribly hard to debug defects due to stale reference pointers in memory (especially with `ContainerAware` services, or things like e.g. PDO connections), as well as high memory consumption, etc.

* **Expected result [PR]:**
  No references to `$precompiled` are left; everything is shut down, cleaned up, and garbage collected upon `$precompiled = NULL;`

#### Proposed solution
1. Hard-code a special behavior for the service ID `service_container`.
1. Remove the self-reference.
1. For now (until next major release), ensure that all `Container` methods work identically to before — instead of throwing exceptions when e.g. trying to set `service_container` to something that isn't `$this`… (which is technically possible right now, but the operation doesn't remotely make sense)

#### API changes
Per (3) above, this PR is backwards-compatible.  However, for the sake of full disclosure, users may experience the following super-micro changes compared to HEAD:

1. The following method no longer exists in a dumped Container, because it is no longer a registered service:

  ```php
      protected function getServiceContainerService()
      {
          throw new RuntimeException('You have requested a synthetic service ("service_container"). The DIC does not know how to construct this service.');
      }
  ```
  But given the exception, you weren't able to call it anyway.
1. The order/position of `service_container` in the array returned by `Container::getServiceIds()` may be different.

  But that array has no particular order to begin with.

  (Only the component's own unit tests are expecting an identical result at times, which is why I added the hard-coded ID via `$id[]` last, instead of initializing it first with `$id` already.)