Add keepalive comment messages, per spec

keepalive.h

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <thread>
+
+#include "index.h"
+
+namespace firesse {
+
+class KeepAlive {
+  public:
+	KeepAlive(const std::chrono::nanoseconds& max_stale, Index* index);
+	void Start();
+	void Stop();
+
+  private:
+	const std::chrono::nanoseconds max_stale_;
+	Index* index_;
+	int shutdown_;
+
+	std::thread thread_;
+};
+
+} // namespace firesse