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)

-Upon reception of SgNB Addition Request from the eNB, the gNB configures pdcp and rlc entities for the list of DRBs provided from the eNB

Fix RFSIMULATOR issues (gNB side) after merge of NR_test_S1U_RRC_PRACH_procedures into NR_RRC_PRACH_procedures

ENDC path switch S1-U: Integrated S1AP E-RAB-Modification indication message at eNB, interfacing with RRC and X2AP to get proper configuration of E-RABs to be modified and GTP-U configuration at gNB

Testing of NR PDCCH/PDSCH with RRC Configuration. added RRC configuration to NR UE. Updates to UE DCI procedures (moved to MAC) and updates to FAPI interfaces for UE. nr_dlsim is functional, added profiling information for gNB TX.