Support Rate Limits for non-repeatable events

[JDGrimes]

This is a follow-up to https://github.com/WordPoints/hooks-api/issues/10.

https://github.com/WordPoints/hooks-api/issues/10#issuecomment-166353137 outlines the technical issues that this presents (related to reversals), which caused us to punt this from the initial version.

The issue is that the event being reversed may have occurred multiple times. The first occurrence would have caused the period to be added to the database, while the later occurrences would be ignored. But we should only remove the period from the database once all of those fires have been reversed.

Feature Request: https://trello.com/c/r3V5ErEo/110-rate-limits-for-all-events

[JDGrimes]

The scenario that we are concerned about is like this:

• User publishes a post. The period starts.
• They publish another post. They aren't awarded points, because the rate limiting kicked in.
• The first post is deleted. The hit is reversed, and their points are removed.

The expected result would be that they would still get points for that period, because they had still triggered the event with it, they just weren't awarded points for the second post because they had been awarded them for the first.

The current behavior would be that the points would be removed, and the second post wouldn't be taken into account at all.

This is tangentially related to queries and retroactivity, since the retroactive API would probably award points in this case. And using queries would be one possible solution here, I suppose.