To allow to send F1 messages to multiple DUs from the CU, we need to
send these messages via different assoc_id (SCTP association ID,
basically akin to a file descriptor). Currently, the assoc_id is stored
in a context at the F1 module and retrieved in
f1ap_itti_send_sctp_data_req().

To facilitate a later change towards indicating (from the RRC) to which
assoc_id (hence, DU) to send a message, we refactor all
CU_send_*()/DU_send_*() functions to take the assoc_id as a parameter to
which to send messages. In this commit, we then pass the assoc_id from
the context. A later commit changes this (at the CU) to pass the
assoc_id from the message to be sent.