dillon1000/react

Don't block deep low pri updates when bailing out

First I fix a bug where host components didn't properly bail out
although this was unobservable.

When we bail out, we need to ensure that we preserve the highest
remaining priority work that is left to do for that subtree.

This still isn't properly handling the case when that work has the
*same* priority as the current one. That work will be flushed the
*next* tick instead of the current pass.

I can't create a test for that yet since I need setState to get to
that state.