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/search.cpp

@@ -341,7 +341,6 @@ void Thread::search() {
 
   bestValue = delta = alpha = -VALUE_INFINITE;
   beta = VALUE_INFINITE;
-  completedDepth = DEPTH_ZERO;
 
   if (mainThread)
   {