This pr adds support for using lightrail for repo emulation when backing filling since it supports multiple collections. community.lexicon.calendar.* is probably the best example. Not everyone will have rsvp or events so this lets you backfill both easily.
You can use lightrail mode by setting a url and a comma separated signal collection. It is it's own url instead of via the relay since it's mostly standalone atm and not a side card to the relay like the current listReposByCollection service.