Slaves run with --dry_run cause builds to be shown as in-progress

[wbell]

When a slave runs with dry_run, the build is marked as in-progress in the ui until it finishes. It stays stuck at the first step until the build is reset at the end of the slave run.

It feels like dry_run shouldn't modify anything in the database, rather than what it does now: start a build, don't record any step information, stop a build.