Connection threads are detached and never joined, so `save_cookies()` ran on
the main thread while they were still serving: a connection writing a
Set-Cookie after that point lost it, and the process then exited and killed the
thread mid-flight. Before the per-connection move this could not happen -- the
single processor saved on its own way out, ordered against all connection work
on one LocalSet. Draining on the live-connection counter restores that ordering.
Scope, honestly: this bounds the loss window rather than closing it. The drain
does not finish early today because a connection's frame reader is parked in a
socket read and never observes `notify_waiters`, so a client that holds a
connection open costs the full 3s deadline and then gets the warning. Making
readers wake on shutdown is a separate change and not #430's business.
Measured: 2ms to shut down with no connections (unchanged), 3005ms with one
held open, against the 10s `docker stop` allows before SIGKILL. Cookies are
persisted in both cases.
Refs #430, #333.
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