OOP + Client module non-blocking migration fix on WP Admin pages

1. Turned the client migration code to object oriented which makes it more structured and extensible for other platforms. It also makes the code modification for developers way easier.
2. After further investigation, the non-blocking migration code was still working inconsistently. We found that it was failing to process the migration when the triggering request was started from WP Admin, while it was migrating successfully if it was a public site-front request. The reason it was failing is permissions; we were not passing the auth cookies with the spawned request. After passing the cookies the migration started to work consistently for all requests.