nr: improve x2/rrc for ENDC

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)

openair2/RRC/LTE/rrc_eNB_S1AP.c

@@ -1751,6 +1751,7 @@ int rrc_eNB_process_S1AP_E_RAB_RELEASE_COMMAND(MessageDef *msg_p, const char *ms
       msg_delete_tunnels_p = itti_alloc_new_message(TASK_RRC_ENB, GTPV1U_ENB_DELETE_TUNNEL_REQ);
       memset(&GTPV1U_ENB_DELETE_TUNNEL_REQ(msg_delete_tunnels_p), 0, sizeof(GTPV1U_ENB_DELETE_TUNNEL_REQ(msg_delete_tunnels_p)));
       GTPV1U_ENB_DELETE_TUNNEL_REQ(msg_delete_tunnels_p).rnti = ue_context_p->ue_context.rnti;
+      GTPV1U_ENB_DELETE_TUNNEL_REQ(msg_delete_tunnels_p).from_gnb = 0;
 
       for(i = 0; i < NB_RB_MAX; i++) {
         if(xid == ue_context_p->ue_context.e_rab[i].xid) {