Runbot Agressive Janitor

This is a module whose aim is to save space and processing power

It checks the runbot build directory for old builds.
It checks by looking at running builds and comparing the with the contents of the build directory. Any extra builds are removed along with associated databases.
It will also do it's best at kill processes which are orphaned.

Additionally, it sets all builds as done when starting up: A big issue with runbot is that when restarting the server, the build objects are still marked as running when they're clearly not.

Credit to @foutoucour who helped out with the implementation, especially the tests.
This line caused me much harm. When running with multiple workers, each worker will call this __init__ method, and clean up the builds. But Odoo does not start all the workers at the same time (I did not go too deep in this), but the net result is: each time a worker starts, the builds are passed to done, and they get cleaned up afterwards by the janitor. 

I propose adding a check on the write_date here, and I'll update my PR on your branch accordingly