The dump stages run nicely now with --last as the date flag, so thta each step looks for the most recent run and updates the data for that run.

However, since we will have two or more cron jobs a month, it's possible that while one run is concluding on one host, other hosts are ready for the next run and start it. They should be able to, since they are idle; however, --last will mean that they run jobs for the previous run's dumps which we don't want, firstly because the run may be of a different type and second for the reasons stated in T126341

What we should do is pass a fixed starting date for each cron job and let that go through to the stages and the worker scripts.