[WIP] Prioritize nodes with more waiting parents when adding new candidates

[acmorrow]

Contributor Checklist:

• [ ] I have created a new test or updated the unit tests to cover the new/changed functionality.
• [x] I have updated CHANGES.txt (and read the README.rst)
• [x] I have updated the appropriate documentation

[acmorrow]

Let's hang back on merging this. I do think there is something to be done along these lines, but I 1) agree that this is at best a partial implementation because it doesn't globally prioritize, and 2) I haven't been able to demonstrate the increased build throughput that I hoped it would provide.

[mwichmann]

I've added a WIP to the title to that end

[bdbaddog]

@acmorrow - is this PR still valid/useful/ updatable with other changes already merged?

[acmorrow]

I still think there is something to this idea, since, given the option, it always seems better to act on nodes with lots of waiting parents preferentially, but without a meaningful performance testing environment for SCons it is hard to show whether it actually improves throughput.