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

该 issue 提到了 mysqli_poll 和 prepared statements，但没有指出源文件或测试；首先在 php-src 中追踪这些入口点，并确定现有的异步轮询如何处理未准备的查询。定义对 prepared statements 的支持行为，并通过回归覆盖验证并行完成。