VEH-34: Use Processing Table to implement a scheduler

Rather than having to create a seperate cron job for each scheduled process or (worse) run everything on everything at each cron interval, it'd be better to have a scheduling system.

By using the processing table and configuration variables to specify the interval that each job should run at, it should be possible to simply queue a cron function in the processing queue. When the job is complete it could then use the configured interval to calculate when it should next run and re-queue itself.