Use timestamp to communicate collection_timestamp of the parent

Don't re-use old self.timestamp when notifying of event. Instead,
re-compute it fresh.

Fixes Kinto#1769.

docs/core/notifications.rst

@@ -110,7 +110,8 @@ Payload
 The :class:`kinto.core.events.ResourceChanged` and :class:`kinto.core.events.AfterResourceChanged`
 events contain a ``payload`` attribute with the following information:
 
-- **timestamp**: the time of the event
+- **timestamp**: the collection timestamp of the ``(parent_id, resource_name)`` pair for
+  which this event was sent
 - **action**: what happened. 'create', 'update' or 'delete'
 - **uri**: the uri of the impacted resource
 - **user_id**: the authenticated user id

kinto/core/resource/__init__.py

@@ -738,7 +738,7 @@ def postprocess(self, result, action=ACTIONS.READ, old=None):
 
         parent_id = self.get_parent_id(self.request)
         self.request.notify_resource_event(parent_id=parent_id,
-                                           timestamp=self.timestamp,
+                                           timestamp=self.model.timestamp(),
                                            data=result,
                                            action=action,
                                            old=old)