Replace client/v3/balancer with standard components: resolver + round_robin LB

This commit significantly reduces volume of custom code
in etcd client v3, while preserving full existing functionality.

bill-of-materials.json

@@ -170,15 +170,6 @@
 			}
 		]
 	},
-	{
-		"project": "github.com/google/uuid",
-		"licenses": [
-			{
-				"type": "BSD 3-clause \"New\" or \"Revised\" License",
-				"confidence": 0.9663865546218487
-			}
-		]
-	},
 	{
 		"project": "github.com/gorilla/websocket",
 		"licenses": [