DConfChangeset: implement dir resets properly

DConfChangeset: implement dir resets properly

If a dir is reset against a DConfChangeset then the result ought to be
that all keys under that dir read as NULL (until such a time as they are
set to a new value).

This is consistent with the (existing) behaviour that a key will read as
NULL if it, itself, was reset.

In order to make that efficient, we create a separate GHashTable to
serve as a cache of all of the directories that have been reset and
iterate it whenever we do a key lookup that doesn't have a direct hit.

We update (and expand) the test case to reflect this new reality -- the
tests actually had a case that relied on the inconsistent behaviour.

https://bugzilla.gnome.org/show_bug.cgi?id=744678