```
__lt__ is the negation of __gt__ by default.
so samples.Timestamp(1, 1) > samples.Timestamp(1, 1) is false
and samples.Timestamp(1, 1) < samples.Timestamp(1, 1) is true.
but  samples.Timestamp(1, 1) < samples.Timestamp(1, 1) should be false too.
add __lt__ func to fix the bug
```

Signed-off-by: weidongkl <weidongkl@sina.com>