Make a couple more things const

src/io/simulator/simulator_handle.cpp

@@ -163,7 +163,7 @@ bool SimulatorHandle::MaybeTickSimulator() {
   return true;
 }
 
-Time SimulatorHandle::Now() {
+Time SimulatorHandle::Now() const {
   std::unique_lock<std::mutex> lock(mu_);
   return cluster_wide_time_microseconds_;
 }

src/io/simulator/simulator_handle.hpp

@@ -169,7 +169,7 @@ class SimulatorHandle {
     cv_.notify_all();
   }
 
-  Time Now();
+  Time Now() const;
 
   template <class D = std::poisson_distribution<>, class Return = uint64_t>
   Return Rand(D distrib) {