frontend/udp: ensure race-free shutdown

[mrd0ll4r]

Fixes #608

New mechanism to shut down cleanly:

1. closing is closed, only once, guarded by the mutex.
2. Socket read deadline is set, which ensures that serve eventually restarts its loop and realizes we're closing.
3. serve closes serveShutdown when it returns.
4. serveShutdown signals Stop that serve has returned and no more calls to f.wg.Add can be made.
   5. f.wg.Wait is called, which should be race-free now.

Hope that make sense.
Also, some cleanup with where f.wg.Add is called.
Also, this should hopefully finally fix the flaky CI and allow us to merge a bunch of PRs, maybe.

[mrd0ll4r]

@jzelinskie 👉 👈