add leveldb support for needle map

This supposedly should reduce memory consumption. However, for tests
with millions of, this shows consuming more memories. Need to see
whether this will work out. If not, later boltdb will be tested.

go/util/http_util.go

@@ -26,12 +26,12 @@ func init() {
 func PostBytes(url string, body []byte) ([]byte, error) {
 	r, err := client.Post(url, "application/octet-stream", bytes.NewReader(body))
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("Post to %s: %v", url, err)
 	}
 	defer r.Body.Close()
 	b, err := ioutil.ReadAll(r.Body)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("Read response body: %v", err)
 	}
 	return b, nil
 }