New flag to FileChannel.force() on each Writer.flush() or not

And disable FileChannel.force() in the case of AtomicFileWriter.
See the committed javadoc for the full why.

core/src/test/java/hudson/util/FileChannelWriterTest.java

@@ -27,7 +27,7 @@ public class FileChannelWriterTest {
     @Before
     public void setUp() throws Exception {
         file = temporaryFolder.newFile();
-        writer = new FileChannelWriter(file.toPath(), StandardCharsets.UTF_8, StandardOpenOption.WRITE);
+        writer = new FileChannelWriter(file.toPath(), StandardCharsets.UTF_8, true, StandardOpenOption.WRITE);
     }
 
     @Test