Inject cosmetic scriptlets and filters synchronously on page loading

[antonok-edm]

We've had a few instances in which websites have updated their code such that a script runs before Brave has a chance to inject a scriptlet from cosmetic filtering. This has caused YouTube and more recently iCloud to break spectacularly.

We should block the page load on these scriptlets being available. It shouldn't cause any significant delay, particularly if we optimize the current code which has a few unnecessary thread jumps.