implement sticky_pattern option in unlearner

I implemented unlearning with sticky ID feature, as proposed in https://github.com/jubatus/jubatus/issues/756#issuecomment-42396255 ([design document](https://gist.github.com/kmaehashi/ff8e0498531698f15119)).

~~WIP: I haven't confirmed that this works on Nearest Neighbor based algorithms (i.e., `recommender` and `anomaly`) with MIX because of blocking issue https://github.com/jubatus/jubatus/issues/806~~
This code does not have strong exception safety. I think that following code is better:

```
// push_front() has no effect when it throws
lru_.push_front(id);
// erase() never throws
lru_.erase(it->second);
```