Create a virtual interface for CappedVisibilitySnapshot, that has a isRecordVisible() API.

Create a callback that returns a pointer to an instance of this interface given an OperationContext* opCtx and a StringData ident.

Pass this callback from cappedTruncateAfter() all the way to WiredTigerRecordStore::Capped::getCursor(). This part doesn't quite work since cappedTruncateAfter() isn't being used, and visibility is a core idea that's implemented by different cursor types.

Move capped_snapshots source files to db/collection_crud.