Figure out how to reject spam kind 5 events

[chmac]

We get kind 5 events, which are deletions, and we accept them because they're one of our accepted kinds. But we seem to be getting a bunch of them, at least I've seen some big ones.

I wonder if we can find a way to reject them. Maybe by checking if they relate to events we already have on the relay, and if not, rejecting them?