Handle more advanced failure scenarios

[hjr3]

Webhooks are fun to think about. A couple more issues off the top of my head:

• Ordering. Since network requests can take variable amounts of time, how do you ensure those two "foo.updated" events are processed in order, or that the receiver can tell their order? Especially something to consider if the webhooks will retry a few times on intermittent failures.
• Consistency. Always a concern in distributed microservice land, but maybe more acute when generating webhooks right as things are updated: if the receiver uses the webhook to make an API request right back into the system, will the API have the same view of the data?
• DDoS. How do you make sure the webhook destination URL is owned by the subscriber? If your system can generate high volumes of webhook traffic, that could be a problem.
• Infinite loops. A silly one, but the user could conceivably point the webhook at a URL of the system that sends the webhook, in such a way that it will cause a new webhookable event to be generated.

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