Replaces #16324.

From the original PR:

Segmented Request was only being used in Discover. It was only actually segmenting requests for old interval based index patterns (and not even for these index patterns in Dashboard). Interval based index patterns are deprecated (cannot be created in 6.0) and will be completely removed in 7.0. The fact that interval based patterns got segmented was an implementation detail of Kibana that is no longer necessary now that ES is smart enough to quickly skip shards that won't return results. The segmented request code creates a huge maintenance burden that prevents us from easily implementing highly requested features like multi-field sorting. This also makes the search fetching behavior more consistent between Discover and Dashboard (which actually relies on the doc table making requests), which will make it easier to extract that logic into a reusable service when we want to rewrite the doc table in React and remove its dependence on Courier.