win32 shadow server slow to exit

[totaamwin32]

Sending it a stop request, it prints Shutting down in response to client request reasonably quickly, but then there's a 5 second delay until we call clean_quit.
Looks like the UI thread is too busy, probably somewhere in cleanup_protocols? flush_then_close?

[Antoine Martin]

It was caused by mdns_updated, added in r22709.

We now use the background worker to avoid the delays: r26886.