Instead of always passing `status_requested` and `lazy_status_request`
in tandem, and requiring one when the other is present, they are now
always passed as a struct `WorkerStatusChangeRequest`. This struct has
both fields required.
`WorkerStatusChangeRequest` already existed, and is now altered by changing
its field `status_requested` to simply `status` -- the fact that it's
requested is already clear from the context.
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