Always report timers/events when worker unparked

[armanbilge]

Fixes #4225.

A parked worker may wake up for various reasons:

1. expired timer or polling event
2. external interruption / unparking, due to availability of external work
3. spurious wakeup

Due to race conditions, a worker may actually wake up for multiple reasons simultaneously. Thus, it should always check first if there are expired timers or polling events, so it can transition to handling those, before considering other cases.

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In #4225, we were able to detect this bug because the WSTP had only one worker.

1. the I/O event completes and the callback reschedules the associated fiber
2. while rescheduling the fiber, the worker notifies the pool that there is work available to steal
3. the pool unparks the one-and-only worker (another thing we should consider tweaking: don't unpark thyself)
4. the worker believes it was unparked due to external work (there is none) and meanwhile doesn't transition to handling its actual local work
5. the worker goes back to sleep until the next timer fires, at which point it properly transitions to handling local work

[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 🤷

[djspiewak]

Nice catch!