修正FileCacheQueueScheduler导致程序不能正常结束和未关闭流

FileCacheQueueScheduler中开启了一个线程周期运行来保存数据但在爬虫结束后没有关闭导致程序无法结束，以及没有关闭io流。

解决方法：
让FileCacheQueueScheduler实现Closable接口，在close方法中关闭线程以及流。
在Spider的close方法中添加对scheduler的关闭操作。

webmagic-core/src/main/java/us/codecraft/webmagic/Spider.java

@@ -374,6 +374,7 @@ public class Spider implements Runnable, Task {
     public void close() {
         destroyEach(downloader);
         destroyEach(pageProcessor);
+        destroyEach(scheduler);
         for (Pipeline pipeline : pipelines) {
             destroyEach(pipeline);
         }

webmagic-extension/src/main/java/us/codecraft/webmagic/scheduler/FileCacheQueueScheduler.java

@@ -12,17 +12,19 @@ import java.util.Set;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.Executors;
 import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 
+
 /**
  * Store urls and cursor in files so that a Spider can resume the status when shutdown.<br>
  *
  * @author code4crafter@gmail.com <br>
  * @since 0.2.0
  */
-public class FileCacheQueueScheduler extends DuplicateRemovedScheduler implements MonitorableScheduler {
+public class FileCacheQueueScheduler extends DuplicateRemovedScheduler implements MonitorableScheduler,Closeable {
 
     private String filePath = System.getProperty("java.io.tmpdir");
 
@@ -43,6 +45,8 @@ public class FileCacheQueueScheduler extends DuplicateRemovedScheduler implement
     private BlockingQueue<Request> queue;
 
     private Set<String> urls;
+    
+    private ScheduledExecutorService flushThreadPool;
 
     public FileCacheQueueScheduler(String filePath) {
         if (!filePath.endsWith("/") && !filePath.endsWith("\\")) {
@@ -94,7 +98,8 @@ public class FileCacheQueueScheduler extends DuplicateRemovedScheduler implement
     }
 
     private void initFlushThread() {
-        Executors.newScheduledThreadPool(1).scheduleAtFixedRate(new Runnable() {
+    	flushThreadPool = Executors.newScheduledThreadPool(1);
+    	flushThreadPool.scheduleAtFixedRate(new Runnable() {
             @Override
             public void run() {
                 flush();
@@ -161,6 +166,12 @@ public class FileCacheQueueScheduler extends DuplicateRemovedScheduler implement
             }
         }
     }
+    
+    public void close() throws IOException {
+		flushThreadPool.shutdown();	
+		fileUrlWriter.close();
+		fileCursorWriter.close();
+	}
 
     private String getFileName(String filename) {
         return filePath + task.getUUID() + filename;