…dual encoder/decoder (sroze)

This PR was merged into the 4.2-dev branch.

Discussion
----------

[Messenger] Uses a messenger serializer, not an individual encoder/decoder

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ø
| License       | MIT
| Doc PR        | Will come

Makes the component-based even simpler.

**Before**
```php
        $encoderDecoder = Serializer::create();
        $middleware = [new SendMessageMiddleware(new SenderLocator([
            Message::class => new AmqpTransport($encoderDecoder, $encoderDecoder, $connection),
        ]))];
```

**After**
```php
       $middleware = [new SendMessageMiddleware(new SenderLocator([
            Message::class => new AmqpTransport(Serializer::create(), $connection),
        ]))];
```

Commits
-------

5b93f5f Uses a messenger serializer, not an individual encoder/decoder

…erializer default id (ogizanagi)

This PR was merged into the 4.2 branch.

Discussion
----------

[FrameworkBundle][Messenger] Restore check for messenger serializer default id

| Q             | A
| ------------- | ---
| Branch?       | 4.2 <!-- see below -->
| Bug fix?      | yes
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets |  https://symfony-devs.slack.com/archives/C9PQ75TV3/p1543590611003500  <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | N/A

From Slack:

> @adamquaile [4:10 PM]
> So, I just updated to 4.2 today, and got this message:
> > The default Messenger serializer cannot be enabled as the Serializer support is not available. Try enabling it or running "composer require symfony/serializer-pack"
>
> In the docs it's stated:
> > In order to use Symfony's built-in AMQP transport, you will need the Serializer Component. Ensure that it is installed with:
>
>But I haven't yet configured AMQP - I'm using my own transport. Should I be getting this exception?

---
This check was removed in #28405, but is actually still necessary to not fail as soon as you can install the Messenger component without the Serializer one installed.

Commits
-------

1cf17c0 [FrameworkBundle][Messenger] Restore check for messenger serializer default id