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`:

```
"my_string_field" : {
  "type": "string",
  "norms": {
    "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`.

Close #4079