Fix a nasty dangling-event bug when using rate-limiting groups When we freed a bufferevent that was in a rate-limiting group and blocked on IO, the process of freeing it caused it to get removed from the group. But removing the bufferevent from the group made its limits get removed, which could make it get un-suspended and in turn cause its events to get re-added. Since we would then immediately _free_ the events, this would result in dangling pointers. Fixes bug 3041007.