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/routers.go

@@ -40,41 +40,24 @@ func registerDistErasureRouters(router *mux.Router, endpointServerPools Endpoint
 
 // List of some generic handlers which are applied for all incoming requests.
 var globalHandlers = []mux.MiddlewareFunc{
-	// filters HTTP headers which are treated as metadata and are reserved
-	// for internal use only.
-	filterReservedMetadata,
-	// Enforce rules specific for TLS requests
-	setSSETLSHandler,
 	// Auth handler verifies incoming authorization headers and
 	// routes them accordingly. Client receives a HTTP error for
 	// invalid/unsupported signatures.
-	setAuthHandler,
+	//
 	// Validates all incoming requests to have a valid date header.
-	setTimeValidityHandler,
-	// Validates if incoming request is for restricted buckets.
-	setReservedBucketHandler,
+	setAuthHandler,
 	// Redirect some pre-defined browser request paths to a static location prefix.
 	setBrowserRedirectHandler,
 	// Adds 'crossdomain.xml' policy handler to serve legacy flash clients.
 	setCrossDomainPolicy,
-	// Limits all header sizes to a maximum fixed limit
-	setRequestHeaderSizeLimitHandler,
-	// Limits all requests size to a maximum fixed limit
-	setRequestSizeLimitHandler,
+	// Limits all body and header sizes to a maximum fixed limit
+	setRequestLimitHandler,
 	// Network statistics
 	setHTTPStatsHandler,
 	// Validate all the incoming requests.
 	setRequestValidityHandler,
-	// Forward path style requests to actual host in a bucket federated setup.
-	setBucketForwardingHandler,
-	// set HTTP security headers such as Content-Security-Policy.
-	addSecurityHeaders,
 	// set x-amz-request-id header.
 	addCustomHeaders,
-	// add redirect handler to redirect
-	// requests when object layer is not
-	// initialized.
-	setRedirectHandler,
 	// Add new handlers here.
 }
 
@@ -104,6 +87,10 @@ func configureServerHandler(endpointServerPools EndpointServerPools) (http.Handl
 	// Add API router
 	registerAPIRouter(router)
 
+	// Enable bucket forwarding handler only if bucket federation is enabled.
+	if globalDNSConfig != nil && globalBucketFederation {
+		globalHandlers = append(globalHandlers, setBucketForwardingHandler)
+	}
 	router.Use(globalHandlers...)
 
 	return router, nil