add round-robin load balancing to service-registry client

[subes]

if multiple service instances are found, add the option to call them load balanced via round-robin. Maybe resilience4j has something for that, though the implementation should be simple enough.

[subes]

Another example with netflix ribbon: https://spring.io/blog/2015/01/20/microservice-registration-and-discovery-with-spring-cloud-and-netflix-s-eureka

[subes]

the server state can be synced by clients just calling each server to update the state, or better use something like jgroups to distribute state more efficiently between servers that know of each other (though would need another registry or fixed IPs for that)