lightningd: expose created_index and updated_index fields.

If you miss a wait event, you can catch up by doing listinvoices and
getting the max of these fields.  It's also a good debugging clue.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_misc.py

@@ -1017,6 +1017,7 @@ def test_cli(node_factory):
                      '         "amount_msat": 123000,',
                      '         "status": "unpaid",',
                      r'         "description": "d\"[]{}",',
+                     '         "created_index": 1',
                      '      }',
                      '   ]',
                      '}']