Use round robin load balancing instead of 'pick first' for gitaly-ruby

Closes #1167 (closed)

This change will make it possible to increase gitaly-ruby's capacity by load-balancing across multiple worker processes. If there's too much traffic coming, just queueing it up does not solve the problem. This change gives us the option to throw extra capacity at the traffic. The real solution will probably be to bring down the traffic and/or shift it to the Go part of Gitaly.