preparation for UE CONTEXT Release from CU

Merge branch 'feature-127-protocol-split' of https://gitlab.eurecom.fr/oai/openairinterface5g into feature-127-protocol-split

Merge branch 'feature-127-protocol-split' of https://gitlab.eurecom.fr/oai/openairinterface5g into feature-127-protocol-split

Merge branch 'feature-127-protocol-split' of https://gitlab.eurecom.fr/oai/openairinterface5g into feature-127-protocol-split

Merge branch 'feature-127-protocol-split' of https://gitlab.eurecom.fr/oai/openairinterface5g into feature-127-protocol-split

Merge branch 'feature-127-protocol-split' of https://gitlab.eurecom.fr/oai/openairinterface5g into feature-127-protocol-split

Conflicts:
	cmake_targets/tools/build_helper

Merge branch 'feature-127-protocol-split' of https://gitlab.eurecom.fr/oai/openairinterface5g into feature-127-protocol-split

* unified CU/DU in one PROTO_AGENT instance since they are symmetric from UDP POV
* delete a lot of unnecessary code
* better error handling
* can reciprocally send data

* peer_addr is now const
* new_link_udp_server() can bind to address or INADDR_ANY
* socket_udp_receive() happens without a loop
* socket_udp_send() happens without a loop
* sending in two packets (first size, then data) is only performed for TCP,
  SCTP, not for UDP anymore (it is unreliable, so we could miss something and
  will receive complete garbage)

RRCConnectionSetupComplete received in CU! (needed to allocate mem_pool for pdcp. remaining transactions on SRB1 and then SRB2 need to use PDCP (DCCH).

UL_RRC_MESSAGE_TRANSFER correctly received in CU. But : it shouldn't go directly to RRC, it has to go through PDCP via pdcp_data_ind