Log slow php-fpm requests

Summary:
Allow to specify in the pillar a slow_delay parameter.
Any request taking longer will be logged with a stacktrace.

If omitted, the delay will be 5s. To disable the feature,
set the value at 0.

Reference:
http://php.net/manual/en/install.fpm.configuration.php

Test Plan: Fire a slow request through MediaWiki or a custom script

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1487