reduce number of middleware handlers (#13546)

- combine similar looking functionalities into single
  handlers, and remove unnecessary proxying of the
  requests at handler layer.

- remove bucket forwarding handler as part of default setup
  add it only if bucket federation is enabled.

Improvements observed for 1kiB object reads.
```
-------------------
Operation: GET
Operations: 4538555 -> 4595804
* Average: +1.26% (+0.2 MiB/s) throughput, +1.26% (+190.2) obj/s
* Fastest: +4.67% (+0.7 MiB/s) throughput, +4.67% (+739.8) obj/s
* 50% Median: +1.15% (+0.2 MiB/s) throughput, +1.15% (+173.9) obj/s
```

cmd/crossdomain-xml-handler.go

@@ -25,29 +25,21 @@ const crossDomainXML = `<?xml version="1.0"?><!DOCTYPE cross-domain-policy SYSTE
 // Standard path where an app would find cross domain policy information.
 const crossDomainXMLEntity = "/crossdomain.xml"
 
-// Cross domain policy implements http.Handler interface, implementing a custom ServerHTTP.
-type crossDomainPolicy struct {
-	handler http.Handler
-}
-
 // A cross-domain policy file is an XML document that grants a web client, such as Adobe Flash Player
 // or Adobe Acrobat (though not necessarily limited to these), permission to handle data across domains.
 // When clients request content hosted on a particular source domain and that content make requests
 // directed towards a domain other than its own, the remote domain needs to host a cross-domain
 // policy file that grants access to the source domain, allowing the client to continue the transaction.
 func setCrossDomainPolicy(h http.Handler) http.Handler {
-	return crossDomainPolicy{handler: h}
-}
-
-func (c crossDomainPolicy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	// Look for 'crossdomain.xml' in the incoming request.
-	switch r.URL.Path {
-	case crossDomainXMLEntity:
-		// Write the standard cross domain policy xml.
-		w.Write([]byte(crossDomainXML))
-		// Request completed, no need to serve to other handlers.
-		return
-	}
-	// Continue to serve the request further.
-	c.handler.ServeHTTP(w, r)
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// Look for 'crossdomain.xml' in the incoming request.
+		switch r.URL.Path {
+		case crossDomainXMLEntity:
+			// Write the standard cross domain policy xml.
+			w.Write([]byte(crossDomainXML))
+			// Request completed, no need to serve to other handlers.
+			return
+		}
+		h.ServeHTTP(w, r)
+	})
 }