[bugfix] Make possible to create sync tcp-connection

[nikserg]

On some systems creating async TCP connection results in "failed to open stream: Resource temporarily unavailable" exception. To solve this, added VAR_DUMPER_ASYNC variable, which is true by default, so won't affect default behavior.

If one faces "failed to open stream: Resource temporarily unavailable" error, he should add VAR_DUMPER_ASYNC=0 to .env file, and problem would be solved.

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony repository. It is used by Composer to allow developers to depend on specific Symfony components.

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