CoveredSnapshots

Snapshot writes a later durable write to the same store covers: a facet of a Store (Store.facet) whose snapshots can then be written without waiting for their own durability.

The node covers a context when its log, the checkpoints its snapshot subscription saves, and every store its snapshots go to are this one store. The subscription saves a checkpoint only once the snapshots it covers are written, so on one database a checkpoint durable after a crash covers snapshots durable with it, and a snapshot the crash lost comes after the last durable checkpoint: the subscription writes it again from the log. A context split over stores is never covered, since there a checkpoint durable in one store says nothing of the snapshots another lost. A store without the facet writes every snapshot durably, as one not covered does.

Internal to Wow: only its runtime and its PostgreSQL store use it; a third-party store may ask for it later.