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/main.cpp

@@ -45,11 +45,11 @@ int main(int argc, char* argv[]) {
   Pawns::init();
   Tablebases::init(Options["SyzygyPath"]);
   TT.resize(Options["Hash"]);
-  Threads.init(Options["Threads"]);
+  Threads.set(Options["Threads"]);
   Search::clear(); // After threads are up
 
   UCI::loop(argc, argv);
 
-  Threads.exit();
+  Threads.set(0);
   return 0;
 }