engine: seal changesets on changes
engine: seal changesets on changes

When we do change operations, make sure we seal our DConfChangeset
before sharing it between threads.

This will ensure it gets sealed in only one thread instead of being
implicitly sealed in two different threads at the same time when each of
them calls dconf_changeset_describe().

https://bugzilla.gnome.org/show_bug.cgi?id=703073

GNOME/dconf,GNOME/dconf,GNOME/dconf,GNOME/dconf