Introduce elapsed_time()

And reformat a bit time manager code.

Note that now we set starting search time in think() and
no more in ThreadPool::start_thinking(), the added delay
is less than 1 msec, so below timer resolution (5msec) and
should not affect time lossses ratio.

No functional change.

src/thread.cpp

@@ -356,8 +356,6 @@ void ThreadPool::start_thinking(const Position& pos, const LimitsType& limits,
                                 StateStackPtr& states) {
   main()->join();
 
-  SearchTime = now(); // As early as possible
-
   Signals.stopOnPonderhit = Signals.firstRootMove = false;
   Signals.stop = Signals.failedLowAtRoot = false;