tracker: fix inefficient usage of GHashTable
tracker: fix inefficient usage of GHashTable

https://bugzilla.gnome.org/show_bug.cgi?id=746974

GNOME/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,GNOME/grilo-plugins,MikePetullo/grilo-plugins,grilofw/grilo-plugins,grilofw/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins