Re-write fetching logic to cache builds locally

I used to always grab /get_master and then
fan out and grab /get_builds for each builder.

Now I just grab /get_master and then request
individual builds from CBE.  If CBE doesn't
have the build data I've taught the script how
to hit the buildbot master directly.

Currently feeder.py should never hit the BB master
as it doesn't deal with active builds (it actively
filters them out).  However soon I will need to teach
it how to deal with active builds for fresher data.