Upon changing the number of threads, make sure all threads are bound

The heuristic to avoid thread binding if less than 8 threads are requested resulted in the first 7 threads not being bound.
The branch was verified to yield a roughly 13% speedup by @CoffeeOne on the appropriate hardware and OS, and an earlier version of this patch tested well on his machine:

http://tests.stockfishchess.org/tests/view/5a3693480ebc590ccbb8be5a
ELO: 9.24 +-4.6 (95%) LOS: 100.0%
Total: 5000 W: 634 L: 501 D: 3865

To make sure all threads (including mainThread) are bound as soon as the total number exceeds 7, recreate all threads on a change of thread number.
To do this, unify Threads::init, Threads::exit and Threads::set are unified in a single Threads::set function that goes through the needed steps.
The code includes several suggestions from @joergoster.

Fixes issue #1312 

No functional change

src/thread.h

@@ -96,9 +96,8 @@ struct MainThread : public Thread {
 
 struct ThreadPool : public std::vector<Thread*> {
 
-  void init(size_t); // No constructor and destructor, threads rely on globals that should
-  void exit();       // be initialized and valid during the whole thread lifetime.
   void start_thinking(Position&, StateListPtr&, const Search::LimitsType&, bool = false);
+  void clear();
   void set(size_t);
 
   MainThread* main()        const { return static_cast<MainThread*>(front()); }