PartitionLineage
class PartitionLineage
Where a log partition comes from: the metadata format 1 reserves for the online splits of 10.x, so that they are an additive change. Each backend stores it with the partition's counter, writes ROOT for every partition a log is created with, and reads it back with EventLog.lineage; nothing acts on it yet.
parent is the partition this one split from, null for a partition the log was created with. When P doubles, partition p splits into p and p + P by the next bit of the slot (Partitioner.SLOTS); p + P names p its parent.
epoch counts the splits that made the partition: 0 for one the log was created with, one more than its parent's for a child.
Fields may be added: construct one with of; the constructor is internal to Wow.