Pull changes from Lix fork

[MagicRB]

This is tracking issue for tracking and evaluating which changes we might want to pull from the Lix fork of buildbot. I recommend only looking at the changes to the __init__.py file here. So far I have identified the following changes:

• use build-local gc-root directory
  This has a good justification, might want to pull it in, but perhaps only for the derivation gcroots
• Fix marking jobs as successful if they never finish evaluating.
  Not sure what this is fixing, need to look into it more
• never build on the coordinator
  Good idea, but must be configurable
• Add build result tracking, schedule newly available builds
  Probably doesn't apply to us, seems like its related to their per derivation buildstep system

[zowoq]

• per arch worker queue 1, 2