[metadata-store] Implement new API for set_metadata() which allows specifying multiple keys to update.
[metadata-store] Implement new API for set_metadata() which allows
specifying multiple keys to update.

MikePetullo/grilo-plugins,vrutkovs/grilo-plugins,MikePetullo/grilo-plugins,MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins,grilofw/grilo-plugins,GNOME/grilo-plugins,kyoushuu/grilo-plugins,grilofw/grilo-plugins,vrutkovs/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,GNOME/grilo-plugins,vrutkovs/grilo-plugins,kyoushuu/grilo-plugins,jasuarez/grilo-plugins,jasuarez/grilo-plugins,MathieuDuponchelle/grilo-plugins,kyoushuu/grilo-plugins