Add Potentially missing bookkeeping for excessDomChildren

[developit]

trafficstars

Just putting up a WIP of what I am thinking is a potential fix for a few issues, by adding in bookkeeping that appears to be missing for excessDomChildren.

[coveralls]

Coverage remained the same at 100.0% when pulling 22f17a20c28f01bb11d444fd26258520f2315991 on potential-fix-remove-old-children into 45328e9a0e734be9d3857c658a7146e297cb156e on master.

[JoviDeCroock]

Do these issues relate to the one where you have this PR for: https://github.com/preactjs/preact/pull/2210 since this actually looks very good to me

[developit]

@JoviDeCroock my hope was that this would make #2210 unnecessary. I should copy the test over and see if it fails here.

[marvinhagemeister]

I can't decide which approach is better, because it's an area I haven't dabbled much with. I do like the test cases in #2210 though! And from your description @developit it seems like this one here may catch more problems than #2210, right?

[JoviDeCroock]

I've tried adding the tests from #2210 but the tests added there fail

[prateekbh]

@JoviDeCroock @developit is the change in Portal tests in #2210 even required while bringing it here?