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/search.cpp

@@ -290,12 +290,11 @@ void MainThread::search() {
       {
           th->maxPly = 0;
           th->rootDepth = DEPTH_ZERO;
-          th->searching = true;
           if (th != this)
           {
               th->rootPos = Position(rootPos, th);
               th->rootMoves = rootMoves;
-              th->notify_one(); // Wake up the thread and start searching
+              th->start_searching();
           }
       }
 
@@ -324,7 +323,7 @@ void MainThread::search() {
   // Wait until all threads have finished
   for (Thread* th : Threads)
       if (th != this)
-          th->join();
+          th->wait_for_search_finished();
 
   // Check if there are threads with a better score than main thread.
   Thread* bestThread = this;