fix(bhm): deliver exit signal reliably (for real)

> However, as it turns out, `crossbeam-channel`'s channels don't drop
> remaining messages until all associated senders *and* receivers are
> dropped. This means the exit signal won't be delivered as long as
> there's at least one `HangMonitorRegister` or
> `BackgroundHangMonitorChan` maintaining a copy of the sender. To work
> around this and guarantee a rapid delivery of the exit signal, the
> sender is wrapped in `Arc`, and only the worker thread maintains a
> strong reference, thus ensuring both the sender and receiver are
> dropped as soon as the worker thread exits.

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