Tigher loop in KeepAlive; skip syscalls if overloaded

keepalive.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <atomic>
 #include <thread>
 
 #include "index.h"
@@ -17,7 +18,11 @@ class KeepAlive {
   private:
 	const std::chrono::nanoseconds max_stale_;
 	Index* index_;
-	int shutdown_;
+
+	// Two shutdown mechanisms, one for if we're in a tight no-syscall loop,
+	// and one for if we're sleeping in poll()
+	int shutdown_fd_;
+	std::atomic<bool> running_ = true;
 
 	std::thread thread_;
 };