Introduce flags to disable force per instance or globally

The goal here is to:
* be able to selectively choose performance over integrity in some very
  specific cases
* be able to globally disable AtomicFileWriter integrity, and basically
  revert to previous behaviour by setting a system property.
  This flag is meant as an emergency one in case something goes very
  wron on some production system.

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, true, StandardOpenOption.WRITE);
+        writer = new FileChannelWriter(file.toPath(), StandardCharsets.UTF_8, true, true,  StandardOpenOption.WRITE);
     }
 
     @Test