PrivateBrowsingConsumers

netwerk/

- httphandler: unused, BAHLEETED

- cache: global observer singleton that watches PB transitions and assorted other prefs

   - on transition, dooms the active cache and potentially enables the disk/offline caches
   - enables the disk cache on pref change if not in PB
   - enables offline cache on pref change if not in PB

- cookie: global cookie service

   - set up private DB state if in private browsing
   - happens on profile-do-change and creation