Allow controller to receive log messages from outside main thread
Our existing callback system doesn't allow this, but actually it would be pretty easy to take care of.
The trick here would be:
- to make it so that the flush_queued_events_event uses the alert_sockets_create() mechanism, so it can be turned on from another thread.
- To remove the check for whether we're in the main thread from control_event_logmsg().
I think this would be pretty simple -- maybe a couple hours of work -- but we should consider it only for 0.3.5, since it has potential to be very destabilizing if we mess it up.
Found while working on #25951 (moved)