Switch basic readiness to sequential readiness

This is the first part of !130 (merged)

This MR fixes problems that the basic readiness model causes to implementations.
Namely, consider the fixed-priority scheduling with basic readiness model. Note that neither fixed-priority nor basic readiness imply the order in which two jobs of the same task should execute. Which means that the generic priority-driven implementation of a scheduler "pick a pending job with the highest priority that is ready and execute it" lacks the essential determinism regarding which job of a task to execute first.
Sequential readiness resolves this issue by stating that only the first pending job of a task is ready.