Function is_online 

pub fn is_online(dir: Uuid, online_dirs: &[Uuid]) -> bool

DirectoryId.isOnline: whether a replica assigned to dir counts as on an online directory of a broker whose online directories are online_dirs.

UNASSIGNED and MIGRATING are online and LOST is offline. An empty online_dirs also counts as online: Kafka reaches it only for a broker that registered before 3.7-IV2 carried log dirs, and it assumes a broker halts once every log directory is down.