WIP. This atttempts to tackle #16959 (comment) and #16743. (Right now there's a failing test, because the way we handle blocks is (I think) slightly wrong; I may try and work on that in a separate branch first. TBD.)

Basically: at present we only call batch.increment() when something async happens if we're not inside a pending boundary, but that's incorrect — it means that a batch is committed before everything resolves. When work inside a pending boundary does resolve, the batch becomes a zombie.

At the same time, we don't handle effects inside pending boundaries correctly. They should be deferred until the boundary (and all its parents) are ready.

This PR attempts to fix that — during traversal, when we exit a pending boundary, any effects that were collected get deferred until the next flush. We also distinguish between batch.#pending (any ongoing async work) and batch.#blocking_pending (any async work that should prevent effects outside pending boundaries from being flushed).

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