Fix a bug which could corrupt auth chains

[kegsay]

Complement tests

Basically, when we use /state_ids in conjunction with /event requests, the /event request can fail causing a partial state to be returned. When we persist the state, we process dependent events first. If we fail to process a dependent event due to missing the auth_event, we didn't update the in-memory event map. This meant that we could incorrectly persist events that depended on dropped events.

Discovered via Chaos testing.

Pull Request Checklist

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