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)

openair3/GTPV1-U/gtpv1u_eNB.c

@@ -1184,7 +1184,10 @@ int gtpv1u_delete_s1u_tunnel(
     "0 GTPV1U_ENB_DELETE_TUNNEL_RESP rnti %x teid %x",
     GTPV1U_ENB_DELETE_TUNNEL_RESP(message_p).rnti,
     teid_eNB);
-  return itti_send_msg_to_task(TASK_RRC_ENB, instanceP, message_p);
+  if (req_pP->from_gnb)
+    return itti_send_msg_to_task(TASK_RRC_GNB, instanceP, message_p);
+  else
+    return itti_send_msg_to_task(TASK_RRC_ENB, instanceP, message_p);
 }