Decrease CDN downloads

[chrmod]

Tasks

Beta Give feedback

1. Ask for Anti-Tracking update.json only once again
2. Store checksum of adblocker engines to prevent unnecessary downloads
3. Prevent TrackerDB partial updates - only full updates can work
4. Check for adblocker updates less often
5. Enable engine compression on the CDN
6. Investigate "raced" HTTP requests - reduce their likelihood if they regularly happen (and does it explain duplicated requests in DevTools and mitm proxy?)

[smalluban]

I suggest splitting the issue into atomic problems, which are still valid, and closing this meta issue, This issue hides the details, and it looks like it is a reason we do nothing with that lately.