Drain response body properly for http connection pool (#6415)

Currently Go http connection pool was not being properly
utilized leading to degrading performance as the number
of concurrent requests increased.

As recommended by Go implementation, we have to drain the
response body and close it.

cmd/update-main.go

@@ -324,7 +324,7 @@ func downloadReleaseURL(releaseChecksumURL string, timeout time.Duration, mode s
 	if resp == nil {
 		return content, fmt.Errorf("No response from server to download URL %s", releaseChecksumURL)
 	}
-	defer resp.Body.Close()
+	defer CloseResponse(resp.Body)
 
 	if resp.StatusCode != http.StatusOK {
 		return content, fmt.Errorf("Error downloading URL %s. Response: %v", releaseChecksumURL, resp.Status)
@@ -471,7 +471,7 @@ func doUpdate(sha256Hex string, latestReleaseTime time.Time, ok bool) (updateSta
 	if err != nil {
 		return updateStatusMsg, err
 	}
-	defer resp.Body.Close()
+	defer CloseResponse(resp.Body)
 
 	// FIXME: add support for gpg verification as well.
 	if err = update.Apply(resp.Body,