Fix cron tasks to be more robust, with test coverage

[bobwhitelock]

Had a succession of issues due to these, most recently #134, with others linked there.

Could pull out a separate class to handle all cron behaviour and test this, or have integration tests for cron tasks. Improving test coverage is part of #13.

Could also refactor how this works at the same time, to better process (one minutely task with manual scheduling of less frequent tasks?). This is related to #100, but don't necessarily need to switch how tasks are run, just make the existing stuff more robust and understandable.