Workers shouldn't schedule local tasks while parked

[armanbilge]

There's possibly a bigger discussion here, which is whether the fact that polling systems schedule tasks on a worker's local queue while the worker is parked violates some invariant of our runtime 🤷

Originally posted by @armanbilge in #4227 (comment)

Daniel S and I talked about this on Discord, and we decided we do want to avoid scheduling tasks while a worker is parked. I'm adjusting the PollingSystem API so it doesn't have to work this way ...