Make a couple more things const

src/io/simulator/simulator_handle.cpp

@@ -45,7 +45,7 @@ void SimulatorHandle::ShutDown() {
   cv_.notify_all();
 }
 
-bool SimulatorHandle::ShouldShutDown() {
+bool SimulatorHandle::ShouldShutDown() const {
   std::unique_lock<std::mutex> lock(mu_);
   return should_shut_down_;
 }

src/io/simulator/simulator_handle.hpp

@@ -93,7 +93,7 @@ class SimulatorHandle {
 
   void ShutDown();
 
-  bool ShouldShutDown();
+  bool ShouldShutDown() const;
 
   template <Message Request, Message Response>
   void SubmitRequest(Address to_address, Address from_address, uint64_t request_id, Request &&request, Duration timeout,