Potential workaround for poor zstd performance

Performance is still not great, but better

internal/decompress/xz.go

@@ -12,3 +12,9 @@ func (x Xz) Reader(r io.Reader) (io.ReadCloser, error) {
 	rdr, err := xz.NewReader(r, 0)
 	return io.NopCloser(rdr), err
 }
+
+func (x Xz) Resetable() bool { return true }
+
+func (x Xz) Reset(old, src io.Reader) error {
+	return old.(*xz.Reader).Reset(src)
+}