Changes:
- no more option to configure eager/lazy loading of the norms (useless now
that orms are disk-based)
- only the `string`, `text` and `keyword` fields support the `norms` setting
- the `norms` setting takes a boolean that decides whether norms should be
stored in the index but old options are still supported to give users time
to upgrade
- setting a `boost` no longer implicitely enables norms (for new indices only,
this is still needed for old indices)
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