Pagination for Zoekt search results

At present we just revert back to Elasticsearch if a user requests page 2. This is a temporary hack while we figure out how to efficiently implement pagination on the Zoekt side. This hack provides a pretty poor UX because the 2nd page of results in Elasticsearch may be completely different to what they would have been in Zoekt since they implement search differently. Even worse, since Zoekt has multiple results per file, it's totally possible Elasticsearch won't even have 2nd page of results so they just get nothing.

This issue will require us to add some way to paginate through Zoekt results via the API and then use that on the GitLab side. This should be possible as it seems there is some reliable way to get the next set of results using the streaming API. Since GitLab won't use the streaming API we may just need to look how it's implemented there and see if we can extend the behaviour into the HTTP similarly to how we expanded the HTTP API in https://github.com/sourcegraph/zoekt/pull/521