Avoid reading old nodecounts in multithreading

Previously if the thread with id zero finished search before any of the other threads had reset their nodecount, the previous nodecount was read, this fixes this behaviour by clearing nodecounts of all threads before starting search

bench 5022919

src/thread.cpp

@@ -18,6 +18,11 @@ void Thread::clear() {
     state.clearHistory();
 }
 
+void Thread::initSearchInfo(SearchTime st) {
+    state.si.clear();
+    state.si.st = st;
+}
+
 void Thread::detach() {
     thread.detach();
 }
@@ -36,9 +41,11 @@ u64 Thread::nodes() {
 
 void ThreadPool::start(Position &pos, SearchTime &st, int depth) {
     // Before starting the threads set all of them to searching, to avoid the mainthread stopping search
-    // while other threads havent been started yet
-    for (auto &t : threads)
+    // while other threads havent been started yet, also reset SearchInfo here, to avoid reading old nodecounts
+    for (auto &t : threads) {
         t.searching.store(true, std::memory_order_relaxed);
+        t.initSearchInfo(st);
+    }
 
     for (auto &t : threads)
         t.start(pos, st, depth);

src/thread.h

@@ -23,6 +23,7 @@ class Thread {
     void join();
     void stop();
     void clear();
+    void initSearchInfo(SearchTime st);
     void detach();
     bool done();
     int id() const;