do not use large buffers if not necessary (#11220)

without this change, there is a performance
regression for small objects GETs, this makes
the overall speed to go back to pre '59d363'
commit days.

cmd/common-main.go

@@ -51,7 +51,7 @@ func init() {
 	logger.RegisterError(config.FmtError)
 
 	rand.Seed(time.Now().UTC().UnixNano())
-	globalDNSCache = xhttp.NewDNSCache(10*time.Second, 3*time.Second)
+	globalDNSCache = xhttp.NewDNSCache(10*time.Second, 10*time.Second)
 
 	initGlobalContext()