func OpenForRead(lg *zap.Logger, dirpath string, snap walpb.Snapshot) (*WAL, error) {
	return openAtIndex(lg, dirpath, snap, false)
}
func OpenForRead(lg *zap.Logger, dirpath string, snap walpb.Snapshot) (*WAL, error) {
	return openAtIndex(lg, dirpath, snap, false)
}
// OpenForRead only opens the wal files for read.
// Write on a read only wal panics.