[Umbrella] New algorithm for resuming interrupted work

*Resuming* is the ability to re-use fibers after they are interrupted by a higher-priority update. Take the following scenario: A component is updated at a normal, async priority. Before the update is finished processing, a higher-priority update is scheduled (let's say it's synchronous, though it could also be a higher-priority async update). The sync update *interrupts* the async update, leaving it unfinished. After the sync update finishes, we go back to processing the interrupted, async update. It's possible, and even likely, that the interrupted work wasn't touched by the sync work and can be *resumed* without starting over completely. This is an important optimization for several async features we have in mind, including error handling, blockers, pre-rendering, and hidden priority. We used to have an implementation of resuming that mostly worked but had some bugs. A few months ago, I spent some time [identifying the bugs using fuzz testing](https://github.com/facebook/react/pull/9952) and fixing them by iterating on the existing algorithm. I eventually got a [version working that passed all the tests](https://github.com/facebook/react/pull/9695). But even this version didn't have all of the features we wanted, and the algorithm seemed inherently flawed. So we decided it would be best to scrap the existing algorithm and revisit resuming in the future. We now believe we have a better idea of how resuming should work. I'm going to split the work into multiple PRs, and use this issue to keep track of our progress. My apologies if some of my descriptions are hard to follow. It can be difficult to describe without resorting to jargon. I'll iterate on this issue as I work. Always reconcile against current child set (#11564) --------------------------------------------------- This is a small refactor that reflects what we already do without resuming: the set we reconcile against is always the current set. In the reverted resuming algorithm, the set we reconcile agains…