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/search.h

@@ -102,7 +102,6 @@ extern volatile SignalsType Signals;
 extern LimitsType Limits;
 extern RootMoveVector RootMoves;
 extern Position RootPos;
-extern TimePoint SearchTime;
 extern StateStackPtr SetupStates;
 
 void init();