Parser: store parameters in a ThreadedList instead of ZoneList.

src/utils.h

Index: src/utils.h
diff --git a/src/utils.h b/src/utils.h
index bd5589cc895196827e55e2ec8b245f087507329e..d1982b8282d39365d9ab09febbf88f40b7f9cf2f 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -1640,9 +1640,31 @@ class ThreadedList final {
     friend class ThreadedList;
   };
 
+  class ConstIterator final {
+   public:
+    ConstIterator& operator++() {
+      entry_ = (*entry_)->next();
+      return *this;
+    }
+    bool operator!=(const ConstIterator& other) {
+      return entry_ != other.entry_;
+    }
+    const T* operator*() const { return *entry_; }
+
+   private:
+    explicit ConstIterator(T* const* entry) : entry_(entry) {}
+
+    T* const* entry_;
+
+    friend class ThreadedList;
+  };
+
   Iterator begin() { return Iterator(&head_); }
   Iterator end() { return Iterator(tail_); }
 
+  ConstIterator begin() const { return ConstIterator(&head_); }
+  ConstIterator end() const { return ConstIterator(tail_); }
+
   void Rewind(Iterator reset_point) {
     tail_ = reset_point.entry_;
     *tail_ = nullptr;