Stockfish 17122610_VandeVondele - new version!

Stockfish, chess engine UCI

Rating JCER = 3389
🔬 Author: Joost VandeVondele 

Timestamp: 1514281242 
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: 

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 

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