Jobset -> JobsetEvals by JobsetEvals.jobset_id

[grahamc]

Instead of referring to the jobset by its project's name and the jobset's name, refer to it by a jobset_id.

• I added a little in-line notes about how to write database migrations.
• I fixed a problem where the make for updating the schema libraries would succeed even if the SQL was invalid.
• I fixed a minor problem where hydra-dev-server was buffering output to stderr/stdout
• The C++ in hydra-evaluator feels a bit heavy on boilerplate, but it seemed like a good way to go. I'm open to doing something different. I wrote this in a few separate commits to make it easier for me to grok.
• The perl patches were surprisingly short and easy. I clicked around, ran the tests, ran some evaluations and builds

[grahamc]

Thanks for the review, @edolstra! I'll merge once it goes green.