release resource

Author: shuwenwei

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/fragment/FragmentInstanceContext.java

@@ -826,6 +826,7 @@ public synchronized void releaseResource() {
     releaseTVListOwnedByQuery();
 
     fileModCache = null;
+    nonExistentModFiles = null;
     dataRegion = null;
     globalTimeFilter = null;
     sharedQueryDataSource = null;

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/fragment/QueryContext.java

@@ -67,7 +67,7 @@ public class QueryContext {
 
   private volatile boolean isInterrupted = false;
 
-  protected final Set<TsFileID> nonExistentModFiles = new CopyOnWriteArraySet<>();
+  protected Set<TsFileID> nonExistentModFiles = new CopyOnWriteArraySet<>();
 
   // referenced TVLists for the query
   protected final Set<TVList> tvListSet = new HashSet<>();