Move ThreadsManager::exit() to d'tor

And add final touches to this long patch series.

All the series has been verified against regression with
20K games at fast TC.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/main.cpp

@@ -55,6 +55,4 @@ int main(int argc, char* argv[]) {
       cerr << "\nUsage: stockfish bench [hash size = 128] [threads = 1] "
            << "[limit = 12] [fen positions file = default] "
            << "[limited by depth, time, nodes or perft = depth]" << endl;
-
-  Threads.exit();
 }