ULSCH scheduler optimization

Summary

The default ULSCH scheduler (schedule_ulsch_rnti) and sub-process are not easy to read and to understand (at least for us). Also, we saw many small optimizations that can be done. This will also make our job on DRX implementation easier. Hence, we propose to try to improve this part of the code.

It concerns (non exhaustiv):

• In eNB_scheduler_ulsch.c
  • rx_sdu()
  • schedule_ulsch()
  • schedule_ulsch_rnti()
• In eNB_scheduler_primitives.c
  • UE_is_to_be_scheduled()
• In pre_processor.c
  • ulsch_scheduler_pre_processor()
  • assign_max_mcs_min_rb()

We will do regular commit to check that our modifications doean't break the code and is compatible with the CI test.