Fix waiting on Windows to be sure I/O events can still come in.

[jemc]

Prior to this commit we were using a mechanism on Windows called WaitForSingleObject when we wanted to put a thread to sleep until we activated a signal to wake it up later.

Now we use WaitForSingleObjectEx instead, so we can be sure that the thread is kept in an "alertable" state if there is an APC (Async Procedure Call) that needs to be run during the wait period.

We use APCs for socket I/O completion callbacks, and they can only arrive on the thread that initiated them, so it's important to allow such callbacks to run even if the scheduler thread is suspended.

Note that the callbacks we use for sockets will not run any Pony code - they will only dispatch a message via the ASIO subsystem, so running those APCs is safe even if the actor has been work-stealed to another thread and is running there. In such a situation, the ASIO event will just go into the actor's queue and they'll process the message later, as normal. We just need to make sure the APC actually will get run, hence we need to ensure even suspended threads will stay "alertable".

Note that this fixes a similar problem to one of the problems that was fixed in PR #3816, wherein some calls to Sleep were migrated to SleepEx for the same reason - to stay "alertable".

[ponylang-main]

Hi @jemc,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 4325.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[jemc]

This passed CI, and is ready for review.