Fix a few things that I discovered when running ts_scriptqueue with the Kafka version of salobj. The main issue is that one of the unit tests is unreliable due to making unwarrented timing assumptions. The fix is simple (see the u/rowen/kafka branch).

Also change ScriptQueue.start to call super().start() first instead of last. This is generally a good idea (but requires ts_salobj 7.1) and is needed to use the package with kafka salobj.