Filter root moves filter before copy to threads

Currently root moves are copied to all teh threads
but are DTZ filtered only in main thread at the
beginning of teh search.

This patch moves the TB filtering before the
copy of root moves fixing issue #679

https://github.com/official-stockfish/Stockfish/issues/679

No bench change.

src/uci.cpp

@@ -108,7 +108,7 @@ namespace {
   // the thinking time and other parameters from the input string, then starts
   // the search.
 
-  void go(const Position& pos, istringstream& is) {
+  void go(Position& pos, istringstream& is) {
 
     Search::LimitsType limits;
     string token;