Protocol: Better Follow/FollowGroup.

[NfNitLoop]

I'm not happy with FollowGroup.

In an effort not to break backward compatibility, I kept the same repeated Follow follows type, and bolted on an optional int32 follow_group field.

That means that implementors have to keep the follows and follow_groups indexes in sync. And if one implementation is lazy, they can become out of sync and break things.

It's complicated enough that I never even implemented it myself.

Ideally we'd just shove the follows into follow-groups so that it's easy to implement. Maybe we keep the "follows" field around too for ungrouped follows? Need to think on this one.