trim the hashes

I tried taking it down to 6, but it fails the tests consistently ...

id.go

@@ -33,5 +33,5 @@ func (id *IdGenerator) Hash() string {
 	h := md5.New()
 	ns := time.Now().UnixNano() + <-id.id
 	io.WriteString(h, fmt.Sprintf("%d", ns))
-	return fmt.Sprintf("%x", h.Sum(nil))
+	return fmt.Sprintf("%x", h.Sum(nil))[:8]
 }