added X2 Reset procedure (partially). Still to be added to top-level when gNB or eNB are stopped or crash (Assert)

Before this commit, when the gNB crashes, the eNB keeps running.
Same, if the eNB crashes, the gNB keeps running.

In a far past when this happened, the other program (the one not
crashing) was forced to exit.

Then this behavior was changed, for some reason.
But the code was not finished, so now we have a system in an inconsistent
state.

So either we accept that the connection between eNB and gNB can break,
that is one of the programs crashes, and we clean the state of the
program that keeps running. But this is a complex work and it will
surely not survive very long, because someone will change something
in the code later that will break this complex behavior.

Or, simpler, we go back to initial behavior, which is: the program that
did not crash does actually exit when the other crashes.

This commit provides the second solution.

It can easily be reverted whenever someone wants to implement the complex
solution.

(cherry picked from commit 2339972ceaeb75fe18c0b2ae9ea69216dd919668)

This commit contains work done by Eurecom and by Benetel.

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)

- Create itti message to tranfer to RRC the information received at eNB through X2 ENDC Setup Request message

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

An NR-capable UE will transmit some NR measurements.
Once received, the whole process on nr-cell addition is started.
It will fail quickly.

Many things are still missing.

What is not handled properly with this commit:
- in x2ap_eNB_handle_sgNB_add_req we hardcode the gNB ID
- nr triggering is done if we receive a measurement but this measurement is
  not analyzed at all
- no timers (T_dc_overall, T_dc_prep) and as is the code fails and then the
  next triggering after UE reconnection will not work (x2 ID are not well
  handled here and there in the code)

Note also that the gNB part has not been looked at all for this commit.

This is work in progress...

Enabling ITTI interactions between RRC and X2AP for the generation of sgb addition request ACK message at the gNB side

ENDC: Reverse the role of eNB and gNB for the ENDC X2 setup procedure. Currently, gNB: slave sending the setup request, eNB:master.

ENDC for 5G NSA support: Basic implementation of x2ap_gNB_generate_ENDC_x2_setup_response() and x2ap_gNB_handle_ENDC_x2_setup_response()

ENDC for 5G NSA support (preparatory work): Basic implementation of x2ap_eNB_generate_ENDC_x2_setup_request() and x2ap_gNB_handle_ENDC_x2_setup_request()

Dual connectivity: Basic implementation of x2ap_eNB_handle_senb_addition_request and x2ap_eNB_generate_senb_addition_request_ack

- basic version of x2ap_eNB_generate_senb_addition_request with dumb values
- skeleton functions:
    - x2ap_eNB_handle_senb_addition_request
    - x2ap_eNB_handle_senb_addition_request_acknowledge
    - x2ap_eNB_handle_senb_addition_request_reject