Eager norms loading options.

Norms can be eagerly loaded on a per-field basis by setting `norms.loading` to
`eager` instead of the default `lazy`, very similarly to fielddata:

```
"my_string_field" : {
  "type": "string",
  "norms": {
    "enabled": true
    "loading": "eager"
  }
}
```

In case this behavior should be applied to all fields, it is possible to change
the default value by setting `index.norms.loading` to `eager`.

There is no such option for terms as #4079 suggested because all terms
implementations in Lucene load the terms dictionary eagerly.

Close #4079