Rename current_time() to now()

Follow C++11 naming conventions.

No functional change.

src/thread.cpp

@@ -417,7 +417,7 @@ void ThreadPool::start_searching(const Position& pos, const LimitsType& limits,
                                  const std::vector<Move>& searchMoves, StateStackPtr& states) {
   wait_for_search_finished();
 
-  SearchTime = Time::current_time(); // As early as possible
+  SearchTime = Time::now(); // As early as possible
 
   Signals.stopOnPonderhit = Signals.firstRootMove = false;
   Signals.stop = Signals.failedLowAtRoot = false;