type metacacheBlockWriter struct {
	wg           sync.WaitGroup
	streamErr    error
	blockEntries int
}

// newMetacacheBlockWriter provides a streaming block writer.
// Each block is the size of the capacity of the input channel.
// The caller should close to indicate the stream has ended.
func newMetacacheBlockWriter(in <-chan metaCacheEntry, nextBlock func(b *metacacheBlock) error) *metacacheBlockWriter {