Create a unified stack for Xenon

Summary: Unifying the PHP and async stacks for Xenon.
Combining the code from the previous Async and PHP stack functions into one.
Eliminating aspets to those functions that we didn't need.
Currently, keeping the async and php stack functions.  Will remove them when
the www side changes land that no longer use them.

Reviewed By: @jano

Differential Revision: D1423082