Manage private browsing mode in content pref service. CPS should be available
in private browsing mode, but should not store informations on disk, and should
clear all informations once the private session ends.
When setting a pref in private mode, it is stored in an in-memory hash table.
When getting a pref, it is retrieved from that hash table if available.
Otherwise, it is retrieved using the standard mechanism. When removing a pref,
it is retrieved from the hash table. The rationale is that in private mode,
it's ok to read a pref from normal database, but not ok to set it.
The in-memory hash table is cleared when leaving the private browsing mode.
When removing a set of preferences (with removeGroupedPrefs or
removePrefsByName), preferences are removed from the in-memory hashtable, *and*
from normal mode database. The rationale is that visiting a website may trigger
setting/getting/removing for a specific preference only. But removing many
prefs at once is the result of an action not associated with a website. For
example, user may wish to delete all its informations. In that case, user
probably expects to not have those informations restored once it leaves private
browsing mode.
