Duck Deux

Will try to reproduce it on Wintermute. Edit: I reproduced it, also with a debug build, but I don't think it's affecting anything. (no idea where to check either since...

I think clock nets don't get decomposed anyway, so this should be OK if it works with `--router_algorithm parallel`.

Rebased this on master: most of the fixes are backported to #2357, which is going to be squashed and rebased too.

Planning to keep this branch as "code storage", divide the changes into a few PRs and do some refactoring in the process. Can rebase this branch as the smaller PRs...

@amin1377 @kimiatkh Any clue about this?