minor #35970 [VarDumper] DumpServer: log whenever a payload is received (ogizanagi)

This PR was merged into the 5.1-dev branch.

Discussion
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[VarDumper] DumpServer: log whenever a payload is received

| Q             | A
| ------------- | ---
| Branch?       | master <!-- see below -->
| Bug fix?      | no
| New feature?  | yes <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | N/A <!-- prefix each issue number with "Fix #", if any -->
| License       | MIT
| Doc PR        | N/A <!-- required for new features -->

Small improvement so you can get a feedback in the console when a dump was received by using `-vv`:

![Capture d’écran 2020-03-05 à 12 02 34](https://user-images.githubusercontent.com/2211145/75975709-4d899d80-5ed9-11ea-942f-e76a62d82218.png)

Commits
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7cfc3ced9d [VarDumper] DumpServer: log whenever a payload is received

src/Symfony/Component/VarDumper/Server/DumpServer.php

@@ -52,6 +52,10 @@ class DumpServer
         }
 
         foreach ($this->getMessages() as $clientId => $message) {
+            if ($this->logger) {
+                $this->logger->info('Received a payload from client {clientId}', ['clientId' => $clientId]);
+            }
+
             $payload = @unserialize(base64_decode($message), ['allowed_classes' => [Data::class, Stub::class]]);
 
             // Impossible to decode the message, give up.