Improve speed for low-volume subscription fanouts

[jsha]

It would be nice if friends could help each other with pileons through the subscription system. In order to do that, actions fanned out through subscriptions would have to take effect in <1 second. Right now that doesn't happen because we don't trigger an actions run immediately after fanning out subscriptions. In part that's because, for large subscription block lists, that would be a performance issue. But we should special-case situations where there are <10 subscribers, and trigger an immediate actions run.