enb_registry: rework handling of eNB property updates
The idea of an enb_event/0 containing the current MME connection state and the associated information (Global-eNB-ID, eNB/MME conn info) looked promising initially, however turned out to be impractical in light of ongoing MME pooling related changes.
* remove type enb_state/0 * rename type enb_event/0 -> enb_prop/0 * rename function enb_event/2 -> enb_update/2 (now private) * enb_prop/0: separate the state from other properties * enb_update/2: accept a list of enb_prop/0 - enb_proplist/0 * add typed notify_*() helpers that group related property updates, ensuring consistency and serving as the sole public call sites * notify_mme_connecting(): explicitly clears mme_conn_info * openapi: EnbItem.state reflects the actual enb_proxy FSM state
