framework: [6.x/7.x] broadcast() method causes memory leak

Like the title says, I ran into a discussion about some memory leaks for beyondcode/laravel-websockets here: https://github.com/beyondcode/laravel-websockets/issues/325

The basic truth is that after some testing, I found out that running broadcast(...) will severely increase the memory for a ReactPHP loop, for instance.

To replicate this:

• create a new Laravel project
• composer require laravel/websockets:"dev-2.x-memory-leak"
• copy the migrations: php artisan vendor:publish --provider="BeyondCode\LaravelWebSockets\WebSocketsServiceProvider" --tag="migrations"
• run them: php artisan migrate
• get to the root of the project and run php artisan websockets:serve --statistics-interval=1
• open an artisan server with php artisan serve
• Go to http://127.0.0.1:8000/laravel-websockets
• Click on Connect
• Watch the memory increase in the console that ran websockets:serve

As explained in https://github.com/beyondcode/laravel-websockets/issues/325#issuecomment-676565842 and https://github.com/beyondcode/laravel-websockets/issues/325#issuecomment-677198100, the problem starts from a broadcast() that causes the leakage.

I have tested even the code within the event. Running broadcast() caused the memory leaks.