etcd: Bump to new 3.4.x version

This moves to the newest etcd release, and also updates the imports to
the new go.etcd.io path. I think this is a bit of a pain, but might as
well get it done.

etcd/interfaces/client.go

@@ -20,8 +20,8 @@ package interfaces
 import (
 	"context"
 
-	etcd "github.com/coreos/etcd/clientv3" // "clientv3"
-	pb "github.com/coreos/etcd/etcdserver/etcdserverpb"
+	etcd "go.etcd.io/etcd/clientv3" // "clientv3"
+	pb "go.etcd.io/etcd/etcdserver/etcdserverpb"
 )
 
 // WatcherData is the structure of data passed to a callback from any watcher.