Code review comment for lp:~michael.nelson/launchpad/db-build-farm-job-model

This branch is part of a pipeline for

https://blueprints.edge.launchpad.net/soyuz/+spec/build-generalisation
https://dev.launchpad.net/LEP/GeneralBuildHistories

In particular, this branch turns BuildFarmJob into a concrete class, while at the same time, continuing support temporarily for classes that still expect this to be an in-memory object.

It is dependent on the pending schema patch in a previous branch.

To test
=======

First update the test db schema (required as the db patch still needs to be updated to remove the old build table):
psql launchpad_ftest_template -f database/schema/pending/michaeln-build-generalisation.sql
bin/py database/schema/security.py -d launchpad_ftest_template

And then:
bin/test -vvt test_buildfarmjob -t doc/build.txt -t test_buildqueue -t test_sourcepackagerecipebuild -t test_translationtemplatesbuildjob

The next stage will be a similar conversion for PackageBuildFarmJob, followed by switching the BinaryPackageBuild from the old build table to the new binarypackagebuild table.