Possible `current_payload` race condition

[dgw]

Once upon a time, I saw a new PR opened fail to message the channel due to a KeyError: 'pull_request', implying that current_payload may have been replaced by a newer one during processing.

I forgot about it, but fortunately it did get written down somewhere so I could rediscover its existence and open an issue.

All of that current_payload nonsense really has to go. Every time I look at this plugin I find more things that desperately need refactoring…