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[Fiber] "Task" priority for error boundaries and batched updates (#8193)

* Refactor scheduling functions and introduce Task priority

There was lots of duplication across all the scheduling functions. I
think we're far enough along that we can start trying to clean some
stuff up.

Also introduces a new priority level provisionally called Task priority.
This is for work that completes at the end of the current tick, after
the current batch of work has been committed. It's different from
Synchronous priority, which needs to complete immediately.

A full implementation of Task priority will follow. It will replace
the current batching solution.

* Implement Task priority

Task priority is similar to Synchronous priority. Both are flushed in
the current tick. Synchronous priority is flushed immediately (e.g. sync
work triggered by setState will flush before setState exits), where as
Task is flushed after the current batch of work is committed.

Currently used for batchedUpdates and nested sync updates. Task should
also be used for componentDidUpdate/Mount and error boundary work. I'll
add this in a later commit.

* Make error boundaries use Task Priority

I have all but one error fixed. Not sure how tricky the last one is,
but I'm cautiously optimistic. Pushing to show my current progress.

* Remove recursion from handleErrors

Changed the algorithm of handleErrors a bit to ensure that boundaries
are not revisited once they are acknowledged.

* Add incremental error boundary test

Discovered an edge case: a noop error boundary will break in incremental
mode unless you explicitly schedule an update.

* Refactor error boundaries in Fiber

* Simplify trapError() calls

The existing logic was written before we had a proper error handling system.

* Remove unnecessary flags

* Prevent performTaskWork recursion

* Be stricter about preventing recursion

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