Reduce service duplication

[raskchanky]

I'm in the middle of creating sessionsrv right now, and so far there's a fair amount of code duplication between originsrv and sessionsrv. Basic things like starting up, running migrations, reading in config files, and setting up a grpc server are all more or less identical, modulo the name of the service. We should either figure out a way to abstract all this away into something more generic, or figure out a way to code-gen it all.

It'd be good to have this stuff in place before creating another service.