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

pkg/handlers/forwarder.go

@@ -33,6 +33,7 @@ type Forwarder struct {
 	RoundTripper http.RoundTripper
 	PassHost     bool
 	Logger       func(error)
+	ErrorHandler func(http.ResponseWriter, *http.Request, error)
 
 	// internal variables
 	rewriter *headerRewriter
@@ -61,6 +62,11 @@ func (f *Forwarder) ServeHTTP(w http.ResponseWriter, inReq *http.Request) {
 		FlushInterval: defaultFlushInterval,
 		ErrorHandler:  f.customErrHandler,
 	}
+
+	if f.ErrorHandler != nil {
+		revproxy.ErrorHandler = f.ErrorHandler
+	}
+
 	revproxy.ServeHTTP(w, outReq)
 }