pub const MIGRATING: Uuid;Expand description
DirectoryId.MIGRATING, Uuid(0, 0): the replica already lives in some
log directory, but nobody recorded which.
Kafka reads a partition record with no directories (every record before
3.7-IV2) as all MIGRATING, and its controller places a new replica on a
broker that registered no log-dir ids, or on any broker below 3.7-IV2, as
MIGRATING.