This commit introduces several enhancements in X2AP and RRC to deal
with ENDC. The main goal is to deal with UE's on/off and with various
errors.

The following work is included:

    - DC prep and DC overall X2 timers
    - X2 messages for UE release and DC timers' timeout

Open problems:

    - what to do when the UE sends an scg Failure? This commit removes
      the UE. And this removal may not be correctly done
    - UE removal in the gNB's PHY layer (a next commit is coming,
      but it is hackish and will need a rework)

-Remaining LTE dependency in RLC add_srb function (not used for NSA setup)
-Rename function for DRB static preconfiguration used in noS1 mode

-Create nr_rrc_pdcp_config_asn1_req function
-Update PDCP DRB addition functions with NR structures instead of LTE

Initially targeting ENDC setup with commercial UE
- Missing nr_rrc_pdcp and nr_rrc_rlc configuration wrt the established DRB
- Remaining to test with user plane data traffic (Downlink and Uplink)