I noticed a deadlock in a test in a in-development PR where during a
shutdown storm of things (from a tsnet.Server.Close), LocalBackend was
trying to call magicsock.Conn.Synchronize but the magicsock and/or
eventbus was already shut down and no longer processing events.
Updates #16369
Change-Id: I58b1f86c8959303c3fb46e2e3b7f38f6385036f1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>