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/lock-rest-server-common_test.go

@@ -40,7 +40,7 @@ func createLockTestServer(t *testing.T) (string, *lockRESTServer, string) {
 		},
 	}
 	creds := globalActiveCred
-	token, err := authenticateNode(creds.AccessKey, creds.SecretKey)
+	token, err := authenticateNode(creds.AccessKey, creds.SecretKey, "")
 	if err != nil {
 		t.Fatal(err)
 	}