Increased heartbeat timeout to 60 seconds as we experienced 51s response time from Rados.

This was a rare occurence:
Tue Jan 29 01:12:51 2019 pid=531 tid=951 op=In BackendRados::lockBackoff(): radosCtx.lock_exclusive() obj=Frontend-ctafrontend-531-20190128-18:02:44-0 duration=51.5958

(One out of 5 operations taking more than 1s in the whole day.

objectstore/AgentHeartbeatThread.hpp

@@ -64,7 +64,7 @@ private:
   std::promise<void> m_exit;
   
   /// The heartbeat update rate.
-  std::chrono::seconds const m_heartRate = std::chrono::seconds(30);
+  std::chrono::seconds const m_heartRate = std::chrono::seconds(60);
   
   /// The logging context
   log::Logger & m_logger;