Small speedup in incremental accumulator updates

Instead of updating at most two accumulators, update all accumluators
during incremental updates. Tests have shown that this change yields a
small speedup of at least 0.5%, and up to 1% with shorter TC.

Passed STC:
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 54368 W: 14179 L: 13842 D: 26347
Ptnml(0-2): 173, 6122, 14262, 6449, 178
https://tests.stockfishchess.org/tests/view/66db038a9de3e7f9b33d1ad9

Passed 5+0.05:
LLR: 2.98 (-2.94,2.94) <0.00,2.00>
Total: 55040 W: 14682 L: 14322 D: 26036
Ptnml(0-2): 303, 6364, 13856, 6664, 333
https://tests.stockfishchess.org/tests/view/66dbc325dc53972b68218ba7

Passed non-regression LTC:
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 57390 W: 14555 L: 14376 D: 28459
Ptnml(0-2): 37, 5876, 16683, 6069, 30
https://tests.stockfishchess.org/tests/view/66dbc30adc53972b68218ba5

closes https://github.com/official-stockfish/Stockfish/pull/5576

No functional change

src/position.cpp

@@ -671,6 +671,7 @@ void Position::do_move(Move m, StateInfo& newSt, bool givesCheck) {
     // our state pointer to point to the new (ready to be updated) state.
     std::memcpy(&newSt, st, offsetof(StateInfo, key));
     newSt.previous = st;
+    st->next       = &newSt;
     st             = &newSt;
 
     // Increment ply counters. In particular, rule50 will be reset to zero later on
@@ -963,6 +964,7 @@ void Position::do_null_move(StateInfo& newSt, TranspositionTable& tt) {
     std::memcpy(&newSt, st, offsetof(StateInfo, accumulatorBig));
 
     newSt.previous = st;
+    st->next       = &newSt;
     st             = &newSt;
 
     st->dirtyPiece.dirty_num               = 0;