This PR was merged into the 4.2 branch.

Discussion
----------

[Messenger] Restore message handlers laziness

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

The `HandlersLocator` introduced in 4.2 currently needs to instantiate all message handlers of the bus in order to be wired, while they were lazily loaded in 4.1 via ContainerHandlerLocator (removed).

Example:
```yaml
framework:
    messenger:
        buses:
            messenger.bus.command: ~
services:
    App\Messenger\:
        resource: '../src/Messenger/*Handler.php'
        tags:
            - { name: messenger.message_handler, bus: messenger.bus.command }
```

```php
namespace App\Messenger;

class BarCommand {}
class BarCommandHandler { public function __invoke(BarCommand $command): void {} }

class FooCommand {}
class FooCommandHandler { public function __invoke(FooCommand $command): void {} }
```
(Dumped `HandleMessageMiddleware` factory) Before:

```php
return $this->privates['messenger.bus.command.middleware.handle_message'] = new \Symfony\Component\Messenger\Middleware\HandleMessageMiddleware(new \Symfony\Component\Messenger\Handler\HandlersLocator(array(
    'App\\Messenger\\BarCommand' => \current(array(0 => array(0 => new \App\Messenger\BarCommandHandler()))),
    'App\\Messenger\\FooCommand' => \current(array(0 => array(0 => new \App\Messenger\FooCommandHandler())))
)));
```

After:
```php
return $this->privates['messenger.bus.command.middleware.handle_message'] = new \Symfony\Component\Messenger\Middleware\HandleMessageMiddleware(new \Symfony\Component\Messenger\Handler\HandlersLocator(array(
    'App\\Messenger\\BarCommand' => new RewindableGenerator(function () {
        yield 0 => ($this->privates['App\Messenger\BarCommandHandler'] ?? ($this->privates['App\Messenger\BarCommandHandler'] = new \App\Messenger\BarCommandHandler()));
    }, 1),
    'App\\Messenger\\FooCommand' => new RewindableGenerator(function () {
        yield 0 => ($this->privates['App\Messenger\FooCommandHandler'] ?? ($this->privates['App\Messenger\FooCommandHandler'] = new \App\Messenger\FooCommandHandler()));
    }, 1)
)));
```

Commits
-------

c608e73 [Messenger] Restore message handlers laziness