Add support for multiple lsif-server endpoints
Created by: efritz
Treat LSIF_SERVER_URL
environment variable the same as searcher and symbol URL values. Each supplied field will be put into the endpoint map (which was luckily already written!). Then, on requests to lsif-server with a routingKey (things that are likely to be cached in-memory), the repo+commit are hashed into the same URL during selection.
This is a pre-step to increasing the replica count of lsif-server.