Disable incorrect runtime assert for ASIO thread shutdown.

[jemc]

When shutting down the ASIO thread and destroying its message queue,
it is possible that there may be some message(s) in flight to it,
meaning that even though we drained the queue before destroying,
the queue may be non-empty by the time we get to destroying it.
Hence, we cannot reliably assert that the queue is non-empty.

However, for actors, we can proved definitively that an actor
is done receiving messages before we destroy its message queue,
so we want to retain the runtime assert for the actor case.

This change updates the assert to be conditional based on
a boolean parameter to the ponyint_messageq_destroy function.

For more context, see this Zulip thread.

[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 4122.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.

[SeanTAllen]

This looks good to merge once release notes are added.