countdownlatch trysetcount bug fix + unit test

according to javadoc, we can't set count if count is not zero.
but if we do latch.trySetCount(0) it updates count and returns false.
this souldn't happen.
Personal style; use it if you want or not :)

I add some white lines to mark the different sections:

 ```
@Test
public void testTrySetCount_whenCountIsNotZero() {
   ICountDownLatch latch = hz.getCountDownLatch(randomString());
   
   latch.trySetCount(10);
   
   assertFalse(latch.trySetCount(20));
   assertFalse(latch.trySetCount(0));
   assertEquals(10, latch.getCount());
}
```

This makes it immediately clear where the do-phase and the verify-phase is.