src: make AtExit callback's per Environment

This commit attempts to address one of the TODOs in
https://github.com/nodejs/node/issues/4641 regarding making the
AtExit callback's per environment, instead of the current global.

bnoordhuis provided a few options for solving this, and one was to
use a thread-local which is what this commit attempts to do.

PR-URL: https://github.com/nodejs/node/pull/9163


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>