Alter "New Identity" to make use of onCacheEntryDoomed if that's still needed

One of our Firefox patches is a horrible hack that makes nsCacheService::EvictEntries synchronous. We did this because Mozilla made it Async before providing an observer to notify us that the eviction took place. This allowed evercookies to persist for a short time after New Identity (see #5715 (closed)).

Apparently, we should now be able to listen to https://developer.mozilla.org/en-US/docs/XPCOM_Interface_Reference/nsICacheListener#onCacheEntryDoomed().

Unless of course "doomed" means "We're really going to delete it soon, we swear. But we'd like to give content a few more chances to access it first, just in case they really wanted to recover their evercookies."