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

この Issue では mysqli_poll と prepared statements が挙げられていますが、ソースファイルやテストは示されていません。まず php-src 内でこれらのエントリポイントを追跡し、既存の非同期ポーリングが未準備クエリをどのように処理しているかを確認してください。prepared statement でサポートする動作を定義し、回帰テストのカバレッジによって並列完了を検証してください。