Add keepalive comment messages, per spec

index.h

@@ -5,12 +5,22 @@
 namespace firesse {
 
 // Track live streams
+// O(1) for Add(), Remove(), and Freshen()
+// Manages this by only supporting insertion of the freshest element,
+// and by requiring an iterator for removal.
+// Avoids allocation and folds in iterators by using an intrusive list
+// inside Stream.
 class Index {
   public:
 	void Add(Stream* stream);
 	void Remove(Stream* stream);
 	void Freshen(Stream* stream);
 
+	// Returns time to sleep until next stalest, or min_stale if none
+	// Only calls callback if stalest is at least min_stale
+	// Handles all locking and marks Stream as fresh after callback
+	std::chrono::nanoseconds WithStalest(std::function<void(Stream*)> callback, const std::chrono::nanoseconds& min_stale);
+
   private:
 	std::recursive_mutex mu_;
 	Stream* freshest_ = nullptr;