Query DSL: Add ids_prefix query and filter to fetch docs by prefix ids

Relative to issue #865

This allows to not have the _id field indexed, but still be able to fetch docs by prefix id. 

The query and filter format could be :

``` javascript
{
    "ids_prefix" : {
        "type" : "my_type",
        "values" : ["1", "4", "10"]
    }
}
```

or

``` javascript
{
    "ids_prefix" : {
        "type" : "my_type",
        "prefix" : "1"
    }
}
```
