Flag critical search tree in hash table

Introducing new concept, saving principal lines into the transposition table
to generate a "critical search tree" which we can reuse later for intelligent
pruning/extension decisions.

For instance in this patch we just reduce reduction for these lines. But a lot
of other ideas are possible.

To go further : tune some parameters, how to add or remove lines from the
critical search tree, how to use these lines in search choices, etc.

STC :
LLR: 2.94 (-2.94,2.94) [0.50,4.50]
Total: 59761 W: 13321 L: 12863 D: 33577 +2.23 ELO
http://tests.stockfishchess.org/tests/view/5c34da5d0ebc596a450c53d3

LTC :
LLR: 2.96 (-2.94,2.94) [0.00,3.50]
Total: 26826 W: 4439 L: 4191 D: 18196 +2.9 ELO
http://tests.stockfishchess.org/tests/view/5c35ceb00ebc596a450c65b2

Special thanks to Miguel Lahoz for his help in transposition table in/out.

Bench: 3399866

src/tt.h

@@ -30,7 +30,8 @@
 /// move       16 bit
 /// value      16 bit
 /// eval value 16 bit
-/// generation  6 bit
+/// generation  5 bit
+/// PvNode      1 bit
 /// bound type  2 bit
 /// depth       8 bit
 
@@ -40,8 +41,9 @@ struct TTEntry {
   Value value() const { return (Value)value16; }
   Value eval()  const { return (Value)eval16; }
   Depth depth() const { return (Depth)(depth8 * int(ONE_PLY)); }
+  bool pv_hit() const { return (bool)(genBound8 & 0x4); }
   Bound bound() const { return (Bound)(genBound8 & 0x3); }
-  void save(Key k, Value v, Bound b, Depth d, Move m, Value ev);
+  void save(Key k, Value v, bool PvNode, Bound b, Depth d, Move m, Value ev);
 
 private:
   friend class TranspositionTable;
@@ -76,7 +78,7 @@ class TranspositionTable {
 
 public:
  ~TranspositionTable() { free(mem); }
-  void new_search() { generation8 += 4; } // Lower 2 bits are used by Bound
+  void new_search() { generation8 += 8; } // Lower 3 bits are used by PV flag and Bound
   TTEntry* probe(const Key key, bool& found) const;
   int hashfull() const;
   void resize(size_t mbSize);