Allow shallower singular extensions

While there, enforce no child node futility pruning for singular
searches

STC:
LLR:  3.04/2.94<0.00, 10.00> Elo diff: 11.07 [3.75, 18.40] (95%)
Games: 2104 W: 295 L: 228 D: 1581 Draw ratio: 75.1%
Pntl: [9, 178, 621, 225, 19]

LTC:
LLR:  2.94/2.94<0.00, 10.00> Elo diff: 5.71 [1.34, 10.07] (95%)
Games: 3655 W: 308 L: 248 D: 3099 Draw ratio: 84.8%
Pntl: [8, 209, 1337, 261, 12]

Bench: 2070693

src/Search.cpp

@@ -474,7 +474,7 @@ namespace Search
         bool improving = !InCheck && Ply >= 2 && data.previous(2) && (data.static_eval > data.previous(2)->static_eval);
 
         // Futility pruning
-        if (!PvNode && depth < 9 && !InCheck && abs(static_eval) < SCORE_TB_WIN_FOUND)
+        if (!PvNode && depth < 9 && !InCheck && !HasExcludedMove && abs(static_eval) < SCORE_TB_WIN_FOUND)
         {
             Score margin = 150 * (depth - improving);
             if (static_eval - margin >= beta)
@@ -565,7 +565,7 @@ namespace Search
             // If all moves fail low then we extend the TT move
             if (!RootSearch && 
                 tt_hit &&
-                depth > 8 &&
+                depth > 4 &&
                 !HasExcludedMove &&
                 move == tt_move &&
                 tt_depth >= depth - 3 &&