Send other client messages through the broadcast queue

[jsdt]

Description of Changes

For context, there is a discussion of potential race conditions here.

The short explanation is that to make sure clients see things in a consistent order, we need to add outgoing messages to the broadcast_queue while we are still holding the associated database transaction lock.

I still need to do this for one-off queries.
This still has an issue with one-off queries, since I haven't routed those messages through.

The change is mostly a matter of changing code like sender.send_message(message) to broadcast_queue.send_client_message(sender, message).

Expected complexity level and risk

3. I don't have a good idea for how to prevent people from accidentally calling sender.send_message when they should be using the broadcast queue, so some bugs might creep back in over time.

Testing

So far I gave this a very basic test with the quickstart-chat app locally.

[gefjon]

I don't have a good idea for how to prevent people from accidentally calling sender.send_message when they should be using the broadcast queue, so some bugs might creep back in over time.

A doc comment on send_message instructing readers to use the broadcast queue instead seems like a good place to start.