When a cluster hasn't run for a while, the default behavior is to keep executing schedules until they are caught up with the present time.
By setting `catch_up` to False, schedules will not play catch up and instead get rescheduled to the future.
The core module was becoming too large.
By splitting it in cluster, monitor and tasks it becomes more manageable. It also reflects the documentation structure.
Workers now share a ctype with the sentinel to indicate if they are currently executing a task. This timer is incremented each guard loop until the worker resets it after finishing a job or until it reaches the TIMEOUT value and the worker is terminated by the sentinel.