use the atomic integer counter to represent the size

modules/elasticsearch/src/main/java/org/elasticsearch/index/translog/memory/MemoryTranslog.java

@@ -60,7 +60,7 @@ public class MemoryTranslog extends AbstractIndexShardComponent implements Trans
     }
 
     @Override public int size() {
-        return operations.size();
+        return operationCounter.get();
     }
 
     @Override public ByteSizeValue estimateMemorySize() {