enb_{proxy,registry}: signal MME conn info on SCTP comm_up
Previously, mme_aid/mme_saddr/mme_sport were only signalled to the enb_registry once the S1 Setup procedure completed. This meant the REST API could not show MME connection details for eNBs stuck in wait_s1setup_rsp state (e.g. due to a slow or retrying MME).
Add notify_mme_comm_up/2, called at SCTP comm_up, which stores the full conn_info (including mme_aid, mme_saddr, mme_sport) in the registry as soon as the SCTP connection is established.
notify_mme_connected/2 is simplified to notify_mme_connected/1: it now only flips the state to 'connected', since conn_info is already stored.
