dillon1000/react

Don't drop updates until they are committed

Restructures the update queue to maintain a pointer to the first
pending update, which solves a few problems:

- Updates that occur during the begin phase (e.g. in cWRP of a child)
aren't dropped, like they are currently. This isn't working yet because
the work priority is reset during completion. The following item will
fix it.
- Sets us up to be able to add separate priorities to each update in
the queue. I'll add this in a subsequent commit.