Rewrite how threads are spawned

Instead of creating a running std::thread and
returning, wait in Thread c'tor that the native
thread of execution goes to sleep in idle_loop().

In this way we can simplify how search is started,
because when main thread is idle we are sure also
all other threads will be idle, in any case, even
at thread creation and startup.

After lazy smp went in, we can simpify and rewrite
a lot of logic that is now no more needed. This is
hopefully the final big cleanup.

Tested for no regression at 5+0.1 with 3 threads:
LLR: 2.95 (-2.94,2.94) [-5.00,0.00]
Total: 17411 W: 3248 L: 3198 D: 10965

No functional change.

src/uci.cpp

@@ -170,7 +170,7 @@ void UCI::loop(int argc, char* argv[]) {
           || (token == "ponderhit" && Search::Signals.stopOnPonderhit))
       {
           Search::Signals.stop = true;
-          Threads.main()->notify_one(); // Could be sleeping
+          Threads.main()->start_searching(true); // Could be sleeping
       }
       else if (token == "ponderhit")
           Search::Limits.ponder = 0; // Switch to normal search
@@ -211,7 +211,7 @@ void UCI::loop(int argc, char* argv[]) {
 
   } while (token != "quit" && argc == 1); // Passed args have one-shot behaviour
 
-  Threads.main()->join(); // Cannot quit whilst the search is running
+  Threads.main()->wait_for_search_finished();
 }