Fix progress issue with shuffling extensions

Fixes issues #2126 and #2189 where no progress in rootDepth is made for particular fens:

8/8/3P3k/8/1p6/8/1P6/1K3n2 b - - 0 1
8/1r1rp1k1/1b1pPp2/2pP1Pp1/1pP3Pp/pP5P/P5K1/8 w - - 79 46

the cause are the shuffle extensions. Upon closer analysis, it appears that in these cases a shuffle extension is made for every node searched, and progess can not be made. This patch implements a fix, namely to limit the number of extensions relative to the number of nodes searched. The ratio employed is 1/4, which fixes the issues seen so far, but it is a heuristic, and I expect that certain positions might require an even smaller fraction.

The patch was tested as a bug fix and passed:

STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 56601 W: 12633 L: 12581 D: 31387
http://tests.stockfishchess.org/tests/view/5d02b37a0ebc5925cf09f6da

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 52042 W: 8907 L: 8837 D: 34298
http://tests.stockfishchess.org/tests/view/5d0319420ebc5925cf09fe57

Furthermore, to confirm that the shuffle extension in this form indeed still brings Elo, one more test at VLTC was performed:
VLTC:
LLR: 2.96 (-2.94,2.94) [0.00,3.50]
Total: 142022 W: 20963 L: 20435 D: 100624
http://tests.stockfishchess.org/tests/view/5d03630d0ebc5925cf0a011a

Bench: 3961247

src/search.cpp

@@ -932,7 +932,7 @@ moves_loop: // When in check, search starts from here
       else if (   PvNode
                && pos.rule50_count() > 18
                && depth < 3 * ONE_PLY
-               && ss->ply < 3 * thisThread->rootDepth / ONE_PLY) // To avoid too deep searches
+               && ++thisThread->shuffleExts < thisThread->nodes.load(std::memory_order_relaxed) / 4)  // To avoid too many extensions
           extension = ONE_PLY;
 
       // Passed pawn extension

src/thread.cpp

@@ -191,7 +191,7 @@ void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
 
   for (Thread* th : *this)
   {
-      th->nodes = th->tbHits = th->nmpMinPly = 0;
+      th->shuffleExts = th->nodes = th->tbHits = th->nmpMinPly = 0;
       th->rootDepth = th->completedDepth = DEPTH_ZERO;
       th->rootMoves = rootMoves;
       th->rootPos.set(pos.fen(), pos.is_chess960(), &setupStates->back(), th);

src/thread.h

@@ -59,7 +59,7 @@ public:
 
   Pawns::Table pawnsTable;
   Material::Table materialTable;
-  size_t pvIdx, pvLast;
+  size_t pvIdx, pvLast, shuffleExts;
   int selDepth, nmpMinPly;
   Color nmpColor;
   std::atomic<uint64_t> nodes, tbHits, bestMoveChanges;