Add 'xenon' module for aggregating ext_xenon-produced traces

Provisions a simple Python daemon that subscribes to Xenon PHP traces published
on a Redis server and writes them to disk, plus a cron job that uses
<https://github.com/brendangregg/FlameGraph> to generate flame graphs from the
data.

Change-Id: I09926c8c26da29baedcf5ab3ef219770690be30a