* Restructure the code handling the UEs F1AP knows
* This fixes a bug that only two phones can be connected

* MAC differentiates between DU/eNB
* sends ITTI message to DU task for F1AP_UE_CONTEXT_RELEASE_REQ
* message is dispatched to correctly in DU_TASK
* sender correctly fills IDs and cause and sends

- add message sender (CU) + handler (CU) and fill with data from ITTI message
- add PLMNID_TO_MCC_MNC and BIT_STRING_TO_TRANSPORT_LAYER_ADDRESS_IPv4 macros
- correct TRANSPORT_LAYER_ADDRESS_TO_BIT_STRING:
  change name to TRANSPORT_LAYER_ADDRESS_IPv4_TO_BIT_STRING, make it send 4B, not 5