Feature request: mysqli_poll async support for prepared statements also

Description

Recently PHP True Async RFC was rejected. It was supposed to have ONE worker and multiple coroutines. But that functionality would be useful only for external calls (like DB, HTTP etc.) that need time to respond.

While for HTTP calls there is curl_multi_init, for parallel DB queries there is no alternative when prepared statements are needed.

So we propose to finish mysqli_poll or implement similar one for running parallel prepared statements.

We already use mysqli_poll to eager load relations in Eloquent that don't have bindings when the number of relations is big (over 20).

https://stackoverflow.com/questions/36358865/mysqli-async-available-with-prepared-statements-in-php

El issue menciona mysqli_poll y las sentencias preparadas, pero no indica archivos fuente ni tests; empieza por rastrear esos puntos de entrada en php-src y establecer cómo el sondeo asíncrono existente gestiona las consultas no preparadas. Define el comportamiento admitido para las sentencias preparadas y verifica la finalización en paralelo con cobertura de regresión.