Fix possible assertion failure in IndicesQueryCache.close (#38714)

The assertion that the stats2 map is empty in
IndicesQueryCache.close has been observed to
fail very occasionally in internal cluster tests.

The likely cause is a cross-thread visibility
problem for a count variable.  This change
makes that count volatile.

Relates #37117

server/src/main/java/org/elasticsearch/indices/IndicesQueryCache.java

@@ -206,7 +206,7 @@ QueryCacheStats toQueryCacheStats() {
     }
 
     private static class StatsAndCount {
-        int count;
+        volatile int count;
         final Stats stats;
 
         StatsAndCount(Stats stats) {