Services

This is an attempt to make custom AsyncMultiService usable inside an advanced master.cfg or a buildbot plugin.

You can start review by looking at the doc in the end of the patch. I hope this explains the idea.

Question: do we stick with the pattern "config is not sent during __init__" but during reconfig? And then how the configuration is sent to the service? How do we describe "send this part of the config" to the reconfigService method?