deposit: duplicate validator with prod inspirehep

One issue, but it's not a problem of the validator itself, rather our import logic: on import, even if there are some validation errors, the id is imported.

depends on: 
https://github.com/inspirehep/inspire-next/pull/105
https://github.com/inveniosoftware/invenio/pull/2433
Thanks to @kaplun he suggested if you just want to see if a record exists (we don't need json for that) you can use

```python
'of': 'id'
```

which is faster to compute and will return you a list of the ids found ```'[1,2,3]'```. If you want to get a python list out of it you can do json.loads() on the returned value.

Can you check that it works and update the code? That will make it faster and simpler