This fixes a race condition where a publisher might send an event, and
immediately after issue a subscription for the same event ID. Prior
to this change, that event would have been published on the broadcast
channel (and ignored by our publisher, because they had not yet issued
the subscription), but not yet committed to the database. Their
subscription would trigger a database query which would return zero
results. Therefore, they would never see the event they published.
The noscl tool is one client that would suffer from this.
Now, all events are broadcast only after they exist in the database,
so a late subscription will always return the event.
