Motivation:

`SelectableEventLoop` uses `ScheduledTask` to track tasks scheduled for some
specific point in time, but also tasks that are supposed to be executed immediately.

This casuse performance issues where the number of immediate tasks grows, as
they need to be kept sorted in the `_scheduledTasks` `PriorityQueue`. Additionally,
that datastructure is protected by lock(s) which casues lock contention from
other threads.

Finally, immediate tasks differ from future scheduled tasks in that they don't have
a `failFn` to handle destruction at the end of `SelectableEventLoop.run()` execution.

Modifications:

* Modify `ScheduledTask` to only track closure tasks and require `failFn` to be provided.
* Retain `UnderlyingTask` for tracking closure or `ErasedUnownedJob` for async-await tasks.
* Add `SelectableEventLoop._immediateTasks` to track immediate tasks (collection of `UnderlyingTask`-s).
* Split production / consumption of the tasks to match the datastructure changes described above.
* Modify the `defer` block of `SelectableEventLoop.run()` to execute immediate tasks, instead of dropping them silently.

Result:

Performance of workloads relying on queuing lots of tasks gets better.

Match 5.10 change made here: apple@e4102ae#diff-660361849e6074a975e1787c5836a6138eef55096f2b80fb76d5da0af73b48d5R40

…bleEventLoop down.

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Co-authored-by: Johannes Weiss <johannesweiss@apple.com>

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…ing-optimisations' into gm-selectableeventloop-job-queueing-optimisations

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…Deque growth properties

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