streamline broadcast to lobbies

[NBKelly]

Marking this is a draft because:

1. I'm not 100% confident in this
2. I want to test it out on playtest for a week or two

So my two insights:

1. If we need to send stuff to 50 or so people, we're currently doing it all on one thread (we typically have a single thread maxed out)
2. people in a game only care about updates that involve them, and (for that little counter in the corner) if a game is created or destroyed

So I think I tackled both of those. Not sure if I did it right. It seems to work locally, but that's not a good measure of anything. I'll throw it up on the playtest server tonight.