metacache: Always close block writer (#10973)

In some cases a writer could be left behind unclosed, leaking compression blocks.

Always close and set compression concurrency to 2 which should be fine to keep up.

cmd/metacache-stream_test.go

@@ -370,6 +370,7 @@ func Test_newMetacacheStream(t *testing.T) {
 	r := loadMetacacheSample(t)
 	var buf bytes.Buffer
 	w := newMetacacheWriter(&buf, 1<<20)
+	defer w.Close()
 	err := r.readFn(func(object metaCacheEntry) bool {
 		err := w.write(object)
 		if err != nil {