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_eNB_decoder.c

@@ -69,6 +69,14 @@ static int x2ap_eNB_decode_initiating_message(X2AP_X2AP_PDU_t *pdu)
         	X2AP_INFO("X2AP_ProcedureCode_id_sgNBReconfigurationCompletion message!\n");
         break;
 
+    case X2AP_ProcedureCode_id_meNBinitiatedSgNBRelease:
+      X2AP_INFO("X2AP_ProcedureCode_id_meNBinitiatedSgNBRelease message!\n");
+      break;
+
+    case X2AP_ProcedureCode_id_sgNBinitiatedSgNBRelease:
+      X2AP_INFO("X2AP_ProcedureCode_id_sgNBinitiatedSgNBRelease message!\n");
+      break;
+
     default:
       X2AP_ERROR("Unknown procedure ID (%d) for initiating message\n",
                   (int)pdu->choice.initiatingMessage.procedureCode);
@@ -103,6 +111,10 @@ static int x2ap_eNB_decode_successful_outcome(X2AP_X2AP_PDU_t *pdu)
     	X2AP_INFO("x2ap_eNB_decode_successfuloutcome_message!\n");
     	break;
 
+    case X2AP_ProcedureCode_id_meNBinitiatedSgNBRelease:
+      X2AP_INFO("meNBinitiatedSgNBRelease successful outcome!\n");
+      break;
+
     default:
       X2AP_ERROR("Unknown procedure ID (%d) for successfull outcome message\n",
                   (int)pdu->choice.successfulOutcome.procedureCode);