improve too many open files implementation

src/main/java/cn/edu/tsinghua/iotdb/query/engine/QueryForMerge.java

@@ -59,7 +59,7 @@ public class QueryForMerge {
 	private void unlockForCurrentQuery() {
 		try {
 			ReadLockManager.getInstance().unlockForOneRequest();
-		} catch (ProcessorException e) {
+		} catch (ProcessorException | IOException e) {
 			e.printStackTrace();
 		}
 	}

src/main/java/cn/edu/tsinghua/iotdb/query/reader/FileReaderMap.java

@@ -10,6 +10,14 @@ public class FileReaderMap {
     /** map to store opened file stream **/
     private static ThreadLocal<Map<String, TsRandomAccessLocalFileReader>> fileReaderMap = new ThreadLocal<>();
 
+    private static class ReaderHolder {
+        private static final FileReaderMap INSTANCE = new FileReaderMap();
+    }
+
+    public static FileReaderMap getInstance() {
+        return ReaderHolder.INSTANCE;
+    }
+
     public static TsRandomAccessLocalFileReader get(String path) throws IOException {
         if (fileReaderMap.get() == null) {
             fileReaderMap.set(new HashMap<>());
@@ -32,9 +40,11 @@ public class FileReaderMap {
                 TsRandomAccessLocalFileReader reader = entry.getValue();
                 reader.close();
             }
+
+            fileReaderMap.get().clear();
         }
 
-        fileReaderMap.get().clear();
-        fileReaderMap.remove();
+
+        //fileReaderMap.remove();
     }
 }

src/main/java/cn/edu/tsinghua/iotdb/query/reader/ReaderManager.java

@@ -95,12 +95,7 @@ public class ReaderManager {
         }
     }
 
-    public void close() throws IOException {
-        for (Map.Entry<String, List<RowGroupReader>> entry : rowGroupReaderMap.entrySet()) {
-            List<RowGroupReader> rowGroupReaderList = entry.getValue();
-            for (RowGroupReader reader : rowGroupReaderList) {
-                reader.close();
-            }
-        }
+    public void close() {
+        rowGroupReaderMap.clear();
     }
 }

src/main/java/cn/edu/tsinghua/iotdb/service/TSServiceImpl.java

@@ -159,7 +159,7 @@ public class TSServiceImpl implements TSIService.Iface, ServerContext {
 		try {
 			ReadLockManager.getInstance().unlockForOneRequest();
 			clearAllStatusForCurrentRequest();
-		} catch (ProcessorException e) {
+		} catch (ProcessorException | IOException e) {
 			LOGGER.error("Error in closeOperation : {}", e.getMessage());
 		}
 		return new TSCloseOperationResp(new TS_Status(TS_StatusCode.SUCCESS_STATUS));