Replaced the existing xpath-heavy system with a more manual traversal
approach. Fixes following slow areas of old system:
- Old system would repeat ID-setting action for elements (Headers could
be processed up to three times).
- Old system had a few very open xpath queries for headers.
- Old system would update links on every ID change, which triggers it's
own xpath query for links, leading to exponential scaling issues.
New system only does one xpath query for links when changes are needed.
Added test to cover.
For #3932
31495758a9