Drop request from event

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).

Start with the follow-up in #945 and trace how events are gathered by resource/parent, including how payload and request are stored. Review kinto/plugins/default_bucket/__init__.py and kinto-signer/kinto_signer/utils.py:171 for fabricated requests. Done requires deciding whether request can be removed or relocated, documenting the implications, and identifying the affected behavior.

Written by the indexing model from the issue text.