// 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 {
	w := metacacheBlockWriter{blockEntries: cap(in)}
	w.wg.Add(1)
	go func() {
		defer w.wg.Done()
		var current metacacheBlock
		var n int