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
9763d079b8