Draft: [REF,FIX] Refactor headerlib to allow ajax append and proper minimization and cache busting

• [REF,FIX] Refactor and fix caches to stop breaking headerlib abstraction

• Make the properties of headerlib private
• Add snapshot support, and getting a diff since last snapshot
• Make the hashes of the js added available to javascript
• Figure out a way to exclude the hashes already present in the js of the page from the current snaphot
• Make a clean API for the above
• Finish !4534
• Finish !4439
• Basic review of CDN handling, like !4570 (comment 1796405777)