jwt: Simplify JWT parsing (#8802)

JWT parsing is simplified by using a custom claim
data structure such as MapClaims{}, also writes
a custom Unmarshaller for faster unmarshalling.

- Avoid as much reflections as possible
- Provide the right types for functions as much
  as possible
- Avoid strings.Join, strings.Split to reduce
  allocations, rely on indexes directly.

cmd/handler-utils.go

@@ -201,7 +201,9 @@ func getReqAccessCred(r *http.Request, region string) (cred auth.Credentials) {
 		if owner {
 			return globalActiveCred
 		}
-		cred, _ = globalIAMSys.GetUser(claims.AccessKey())
+		if claims != nil {
+			cred, _ = globalIAMSys.GetUser(claims.AccessKey)
+		}
 	}
 	return cred
 }