[php-src] Issue #20640: mysqli_poll async support for prepared statements also

Issue: https://github.com/php/php-src/issues/20640
Author: macropay-solutions

### Description

Recently PHP True Async RFC was rejected. It was supposed to have ONE worker and multiple coroutines. But That situation 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