src: rename req_wrap with -async/-sync suffix

This commit renames the req_wrap variable to use an -async/-sync
suffix to avoid cases where the variables were being shadowed.

PR-URL: https://github.com/nodejs/node/pull/19628
Refs: https://github.com/nodejs/node/pull/19614


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>