Accumulator cache bugfix and cleanup

STC:
https://tests.stockfishchess.org/tests/view/663068913a05f1bf7a511dc2
LLR: 2.98 (-2.94,2.94) <-1.75,0.25>
Total: 70304 W: 18211 L: 18026 D: 34067
Ptnml(0-2): 232, 7966, 18582, 8129, 243

1) Fixes a bug introduced in
   https://github.com/official-stockfish/Stockfish/pull/5194. Only one
   psqtOnly flag was used for two perspectives which was causing
   wrong entries to be cleared and marked.
2) The finny caches should be cleared like histories and not at the
   start of every search.

closes https://github.com/official-stockfish/Stockfish/pull/5203

No functional change

src/search.cpp

@@ -147,9 +147,6 @@ Search::Worker::Worker(SharedState&                    sharedState,
 
 void Search::Worker::start_searching() {
 
-    // Initialize accumulator refresh entries
-    refreshTable.clear(networks);
-
     // Non-main threads go directly to iterative_deepening()
     if (!is_mainthread())
     {
@@ -506,6 +503,8 @@ void Search::Worker::clear() {
 
     for (size_t i = 1; i < reductions.size(); ++i)
         reductions[i] = int((20.14 + std::log(size_t(options["Threads"])) / 2) * std::log(i));
+
+    refreshTable.clear(networks);
 }