Plugin can trigger reestablishment (on DU)
and show counters of reestablishment (on CU)

For nr_DRB_preconfiguration() and fill_default_secondaryCellGroup(),
there is duplicated code to generate and RLC-BearerConfig, when calling
the existing get_DRB_RLC_BearerConfig() (with the right parameters)
yields a similar configuration (see exceptions below).

Differences:
- We do not support reestablishmentRLC, so we don't set it
- Set prioritisedBitrate to 8kbps instead of infinity (we don't
  prioritize anything, anyway)
- bucketSizeDuration is 50ms instead of 100ms

During the Initial UL RRC Message, the CellGroup is created. Since the
CellGroup specifies the SRB1, the SRB1 should also be created (because
the DL RRC Messages is sent via SRB1). However, before this commit, the
CellGroup is created while having the RLC lock, so we cannot create
SRB1, as it results in a deadlock.

Therefore, we now first create CellGroup, SRB0, and SRB1, before giving
the UE message to the RLC.

Introduce the function nr_rlc_activate_avg_time_to_tx() to
activate/deactivate collection of avg time to tx stats (which
may have an impact on realtime processing, so better not compute
them when not needed).

Fix a little bug in time_average_get_average() for when there is not stat.

-For the moment SRBs are preconfigured (as was done for DRBs in noS1 mode)
but this should be removed

- do DRB preconfiguration for deafult C-RNTI if !do_ra
- moved redundant configuration code at RLC layer to new RLC functions