Thread code reformat

Simplify out low level sync stuff (mutex
and friends) and avoid to use them directly
in many functions.

Also some renaming and better comment while
there.

No functional change.

src/ucioption.cpp

@@ -39,7 +39,7 @@ namespace UCI {
 void on_clear_hash(const Option&) { Search::clear(); }
 void on_hash_size(const Option& o) { TT.resize(o); }
 void on_logger(const Option& o) { start_logger(o); }
-void on_threads(const Option&) { Threads.read_uci_options(); }
+void on_threads(const Option& o) { Threads.set(o); }
 void on_tb_path(const Option& o) { Tablebases::init(o); }