Refactor how the internal scheduled tasks set their timeouts

[alecthomas]

Currently each scheduled task returns a duration which determines when the task will run next. This is flexible, but in practice we never use this, always returning a single value. It also prevents us from easily having separate schedules for production and development, and a production schedule is rarely convenient for development, where we favour responsiveness.

We should remove the ability of tasks to return their own schedule, and instead specify the duration when they're scheduled.