Refactor some threads related code.

This is a code style change that moves some pure thread code into the threads class.

It is a bit more code, but it makes search.cpp cleaner and easier to read by hiding some thread specific functionality.

STC (SMP)
LLR: 2.93 (-2.94,2.94) {-1.50,0.50}
Total: 75896 W: 12073 L: 12026 D: 51797
Ptnml(0-2): 828, 8224, 19872, 8121, 903
https://tests.stockfishchess.org/tests/view/5ed492e8f29b40b0fc95a74c

closes https://github.com/official-stockfish/Stockfish/pull/2720

No functional change.

src/thread.h

@@ -109,6 +109,9 @@ struct ThreadPool : public std::vector<Thread*> {
   MainThread* main()        const { return static_cast<MainThread*>(front()); }
   uint64_t nodes_searched() const { return accumulate(&Thread::nodes); }
   uint64_t tb_hits()        const { return accumulate(&Thread::tbHits); }
+  Thread* get_best_thread() const;
+  void start_searching();
+  void wait_for_search_finished() const;
 
   std::atomic_bool stop, increaseDepth;