Updates the tokio task that monitors `shutdown_notify` and server
requests to ensure that `server.unblock()` is always called, which means
that `ShutdownHandle` only has to invoke `notify_waiters()`.
Now `LoginServer` no longer has to maintain a reference to `Server`. The
`Arc<Server>` only has two active references: the `thread::spawn()` for
reading server messages and the `tokio::task()` that consumes them (and
the shutdown message). Now when shutdown is called (or if login
completes successfully), the `server.unblock()` call ensures the thread
terminates cleanly, which in turn ensures `rx.recv()` in the
`tokio::spawn()` returns `Err`, causing the `tokio::task()` to exit
cleanly, as well.
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2398).
* #2399
* __->__ #2398
* #2396
* #2395
* #2394
* #2393
* #2389
146985f3ff