Kibana often highlights *everything* like this:
```
POST /_search
{
"query": ...,
"size": 500,
"highlight": {
"fields": {
"*": { ... }
}
}
}
```
This can get slow when there are hundreds of mapped fields. I tested
this locally and unscientifically and it took a request from 20ms to
150ms when there are 100 fields. I've seen clusters with 2000 fields
where simple search go from 500ms to 1500ms just by turning on this sort
of highlighting. Even when the query is just a `range` that and the
fields are all numbers and stuff so it won't highlight anything.
This speeds up the `unified` highlighter in this case in a few ways:
1. Build the highlighting infrastructure once field rather than once pre
document per field. This cuts out a *ton* of work analyzing the query
over and over and over again.
2. Bail out of the highlighter before loading values if we can't produce
any results.
Combined these take that local 150ms case down to 65ms. This is unlikely
to be really useful when there are only a few fetched docs and only a
few fields, but we often end up having many fields with many fetched
docs.
b8e9a7125f