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/X2AP/x2ap_ids.c

@@ -70,6 +70,16 @@ int x2ap_find_id_from_id_source(x2ap_id_manager *m, int id_source)
   return -1;
 }
 
+int x2ap_find_id_from_id_target(x2ap_id_manager *m, int id_target)
+{
+  int i;
+  for (i = 0; i < X2AP_MAX_IDS; i++)
+    if (m->ids[i].rnti != -1 &&
+        m->ids[i].id_target == id_target)
+      return i;
+  return -1;
+}
+
 int x2ap_find_id_from_rnti(x2ap_id_manager *m, int rnti)
 {
   int i;
@@ -107,6 +117,16 @@ void x2ap_set_reloc_overall_timer(x2ap_id_manager *m, int ue_id, uint64_t time)
   m->ids[ue_id].tx2_reloc_overall_start = time;
 }
 
+void x2ap_set_dc_prep_timer(x2ap_id_manager *m, int ue_id, uint64_t time)
+{
+  m->ids[ue_id].t_dc_prep_start = time;
+}
+
+void x2ap_set_dc_overall_timer(x2ap_id_manager *m, int ue_id, uint64_t time)
+{
+  m->ids[ue_id].t_dc_overall_start = time;
+}
+
 int x2ap_id_get_id_source(x2ap_id_manager *m, int ue_id)
 {
   return m->ids[ue_id].id_source;