add a unique buildbot run identifier

I haven't tried this on a buildbot yet but I'd like to get a feedback sooner than later.

The main motivation for the change is that it is pretty hard to query data for a single buildbot run. By having an extra column in the builds table we can enforce a unique buildbot run id. Uniqueness of the buildbot_run_id is enforced by buildbot (configuration).

Note: pull requests also adds the column to testresults, but that's probably a premature optimization for now. I'm going to remove it unless somethings it's okay/better to duplicate the information in tests results too. Otherwise there is already a pointer from testresults to build row.

The "pushplan" for this change is probably going to be something like the below:

ALTER TABLE builds ADD COLUMN buildbot_run_id varchar(16);
ALTER TABLE builds ADD INDEX (buildbot_run_id);
(and same thing for testresults table if we decide to keep it)