refactoring and extending the support for FIFO and RR priority scheduling

• reworked the PriorityScheduler (FIFO)
• reworked the PriorityRoundRobinScheduler (RR) which now extends the PriorityScheduler with timeslicing and budgeting related features.
• added a new scheduler which extends the PriorityRoundRobinScheduler to offer both FIFO and RR scheduling policies that can be set per task.