Develop etcdv3 service registry plugin

[bkrahmer]

It looks like the etcdv3 library may be (or nearly is) feature-complete enough to develop a service-registry plugin.

As part of this work, the existing RegistrationManager / RegistrationMonitorWorker should be refactored and generalized. These two classes can retain their functionality, but should be more generic to allow multiple kinds of plugins like consul and etcd.