Refactor DLSCH scheduler

Refactor the DLSCH post processor similarly to what has been done in
!3521  with the goal of saving an additional iteration through all UEs,
hopefully making the DLSCH scheduler faster.

It further includes changes to make working with and debugging multi-UE
operation easier:

- add some measurements in L1 (for total TX/RX time)
- a new option MACRLCs.[0].stats_max_ue to limit the number of UEs in
  the periodic output (default: 8, 0=disable)
- avoid stack overflow in the MAC
- fix the writing of stats to nrMAC_stats.log and nrL1_stats.log when
  the written output reduces by truncating the files
- avoid to schedule retransmission with 0 RBs.