send broadcast messages in sequence (not parallel)

context

• a series of experiments with healtchecks have yielded evidence that sending batches of messages sequentially rather than in parallel can dramatically reduce the overall transmission time for the same number of messages (from ~30sec to ~500ms)
• we don't yet understand the cause, but have observed the outcome consistently enough that we want to introduce the same pattern with our broadcast messages to see if it provides a decent stopgap solution for message lag while we are working on longer term improvements to concurrency in signald