fix: set request ID in tracing context key (#17602)

Since `addCustomerHeaders` middleware was after the `httpTracer`
middleware, the request ID was not set in the http tracing context. By
reordering these middleware functions, the request ID header becomes
available. We also avoid setting the tracing context key again in
`newContext`.

Bonus: All middleware functions are renamed with a "Middleware" suffix
to avoid confusion with http Handler functions.

cmd/crossdomain-xml-handler_test.go

@@ -29,7 +29,7 @@ import (
 func TestCrossXMLHandler(t *testing.T) {
 	// Server initialization.
 	router := mux.NewRouter().SkipClean(true).UseEncodedPath()
-	handler := setCrossDomainPolicy(router)
+	handler := setCrossDomainPolicyMiddleware(router)
 	srv := httptest.NewServer(handler)
 
 	resp, err := http.Get(srv.URL + crossDomainXMLEntity)