rafthttp: move mu to the top in urlPicker struct

mutex protects all the fields.

rafthttp/urlpick.go

@@ -22,8 +22,8 @@ import (
 )
 
 type urlPicker struct {
+	mu     sync.Mutex // guards urls and picked
 	urls   types.URLs
-	mu     sync.Mutex
 	picked int
 }