Reroute requests based token heal/listing (#9939)

When manual healing is triggered, one node in a cluster will 
become the authority to heal. mc regularly sends new requests 
to fetch the status of the ongoing healing process, but a load 
balancer could land the healing request to a node that is not 
doing the healing request.

This PR will redirect a request to the node based on the node 
index found described as part of the client token. A similar
technique is also used to proxy ListObjectsV2 requests
by encoding this information in continuation-token

cmd/handler-utils.go

@@ -1,5 +1,5 @@
 /*
- * MinIO Cloud Storage, (C) 2015, 2016, 2017 MinIO, Inc.
+ * MinIO Cloud Storage, (C) 2015-2020 MinIO, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -452,3 +452,29 @@ func getHostName(r *http.Request) (hostName string) {
 	}
 	return
 }
+
+// Proxy any request to an endpoint.
+func proxyRequest(ctx context.Context, w http.ResponseWriter, r *http.Request, ep ProxyEndpoint) (success bool) {
+	success = true
+
+	f := handlers.NewForwarder(&handlers.Forwarder{
+		PassHost:     true,
+		RoundTripper: ep.Transport,
+		ErrorHandler: func(w http.ResponseWriter, r *http.Request, err error) {
+			success = false
+			w.WriteHeader(http.StatusBadGateway)
+		},
+		Logger: func(err error) {
+			logger.LogIf(GlobalContext, err)
+		},
+	})
+
+	r.URL.Scheme = "http"
+	if globalIsSSL {
+		r.URL.Scheme = "https"
+	}
+
+	r.URL.Host = ep.Host
+	f.ServeHTTP(w, r)
+	return
+}