search: indexed config API supports multiple repo query arguments
Created by: keegancsmith
This converts the API into a batch API by writing a JSON blob per "repo" query parameter in the order they are specified.
Each zoekt indexserver currently queries this endpoint with a hardcoded concurrency of 32. This can cause many many HTTP requests, which on a linux machine without tuning can lead to socket errors. By converting this into a batch API we reduce the number of connections as well as moving the responsbility of concurrency to the frontend which has more knowledge of load/etc.
We also remove backwards compatibility support where repo wasn't specified. Repo has been specified for multiple releases now.