Drop request from event

[glasserc]

This is a follow-up to #945. Right now events are gathered by resource/parent. When this happens, payload becomes essentially meaningless. At best we provide "one of" the payloads that makes sense given the impacted_records. The same is true for request. Why do we need to provide one of these for each event? Under what circumstances does request vary for different impacted_records across events in a request, or within an event group in the same resource/parent? If there are such circumstances, should we move request into each impacted_record? If not, can we stop storing request for each event, since we already have the request when we're processing its events? This is a sincere question -- my inclination is to deprecate payload and request, but I know there are a bunch of places where we fabricate requests for reasons and I haven't thought through the implications of what that means for events (e.g. https://github.com/Kinto/kinto/blob/master/kinto/plugins/default_bucket/__init__.py and https://github.com/Kinto/kinto-signer/blob/master/kinto_signer/utils.py#L171).