[hotfix] actually send broadcast messages in sequence
- we introduced a fix a long time ago to send broadcast messages in sequence on the theory that (1) this would jam up signald's concurrency bugs less and (2) we had observed health checks fail less and transmit faster when sent in sequence
- however, we did not notice that
broadcastMessagewas only ever being called with one recipient (presumably to get per-recipient language choices honored) so we were never actually sending such messages in sequence - @mari has a more comprehensive fix coming in #329 (closed), but until it lands, here is a small patch to make sure broadcast messages actually go out in sequence so we can measure if this makes any kind of difference in perf/reliability