Bring etcd support for bucket DNS federation

- Supports centralized `config.json`
- Supports centralized `bucket` service records
  for client lookups
- implement a new proxy forwarder

cmd/routers.go

@@ -49,6 +49,9 @@ func registerDistXLRouters(router *mux.Router, endpoints EndpointList) {
 var globalHandlers = []HandlerFunc{
 	// set HTTP security headers such as Content-Security-Policy.
 	addSecurityHeaders,
+	// set Bucket forwarding handler to proxy path style requests
+	// when federated backend is enabled.
+	setBucketForwardingHandler,
 	// Ratelimit the incoming requests using a token bucket algorithm
 	setRateLimitHandler,
 	// Validate all the incoming paths.