JimFS doesn't handle file locks in documented way (OverlappingFileLockException)

If I try to lock regular file then locking it the second time will raise OverlappingFileLockException as described in [FileChannel documentation](https://docs.oracle.com/javase/7/docs/api/java/nio/channels/FileChannel.html)

JimFS allows to lock the same file multiple times. Here is a test code

```java
public void testJimfsLock() throws IOException {

FileSystem jimFs = Jimfs.newFileSystem();
FileSystem fs = FileSystems.getDefault();

checkLock(jimFs.getPath("/lockfile"));
checkLock(fs.getPath("/tmp/lockfile"));

}

private void checkLock(Path lockFilePath) throws IOException {
System.out.println("lockFilePath=" + lockFilePath);
FileChannel fileChannel = FileChannel.open(lockFilePath, StandardOpenOption.CREATE, StandardOpenOption.WRITE);
FileLock lock1 = fileChannel.lock();
FileLock lock2 = null;
try {
lock2 = fileChannel.tryLock();
} catch (OverlappingFileLockException e) {
System.out.println("got OverlappingFileLockException");
}

System.out.println("lock1=" + lock1);
System.out.println("lock2=" + lock2);
}
```

It will print the following output:
```
lockFilePath=/lockfile
lock1=com.google.common.jimfs.JimfsFileChannel$FakeFileLock[0:9223372036854775807 exclusive valid]
lock2=com.google.common.jimfs.JimfsFileChannel$FakeFileLock[0:9223372036854775807 exclusive valid]
lockFilePath=/tmp/lockfile
got OverlappingFileLockException
lock1=sun.nio.ch.FileLockImpl[0:9223372036854775807 exclusive valid]
lock2=null
```

As you see attempting to lock file for the second time on regular file system leads to OverlappingFileLockException.