Refactor SCTP association response handler

* this function is used by both CU-CP and CU-UP
* it behaves differently depending on the type of E1 instance:
  CU-CP: inform RRC of E1 connection loss with CU-UP
  CU-UP: trigger a new SCTP association request by sending an ITTI to the CU-UP task
* after receiving the E1AP register request, the CU-UP will send a new association request to
  SCTP until new E1 connection is achieved by sending a TIMER_HAS_EXPIRED message to the CU-UP
  task. The function e1apHandleTimer() is basically triggering a new E1 association request
  until new E1 connection is achieved
* print SCTP association state in e1ap.c whenever needed

Related to #878

openair3/SCTP/sctp_eNB_itti_messaging.c

@@ -68,7 +68,7 @@ int sctp_itti_send_association_resp(task_id_t task_id,
                                     instance_t instance,
                                     sctp_assoc_t assoc_id,
                                     uint16_t cnx_id,
-                                    enum sctp_state_e state,
+                                    sctp_state_e state,
                                     uint16_t out_streams,
                                     uint16_t in_streams)
 {

openair3/SCTP/sctp_eNB_itti_messaging.h

@@ -35,7 +35,7 @@ int sctp_itti_send_association_resp(task_id_t task_id,
                                     instance_t instance,
                                     sctp_assoc_t assoc_id,
                                     uint16_t cnx_id,
-                                    enum sctp_state_e state,
+                                    sctp_state_e state,
                                     uint16_t out_streams,
                                     uint16_t in_streams);