online model copy in classifier

Now, Jubatus's classifier is fragile in case of adding machine or missing `put_diff RPC`.

I improved this problem with a kind of versioning method.

- classifier model should contain a version number.
- when mixing `diff`, it check the versions and mix only newest `diff`
- on `put_diff` compare the version number between `diff` and model.
- if model is obsolete in version, that process fall down to `model getting state`
- `model getting state` process doesn't get RPC from proxy, so the results of analysis will be more precise.