Ensure that we use constants everywhere (#7845)

This allows for canonicalization of the strings
throughout our code and provides a common space
for all these constants to reside.

This list is rather non-exhaustive but captures
all the headers used in AWS S3 API operations

cmd/api-datatypes.go

@@ -20,11 +20,6 @@ import (
 	"encoding/xml"
 )
 
-const (
-	// Response request id.
-	responseRequestIDKey = "x-amz-request-id"
-)
-
 // ObjectIdentifier carries key name for the object to delete.
 type ObjectIdentifier struct {
 	ObjectName string `xml:"Key"`