fetcher: Rework reference counting
fetcher: Rework reference counting

Have OstreeFetcherPendingURI be the GTask's task_data and pass the GTask
around in queues and callback closures.  The reference counting before
was a little confusing and this helps clarify it, at least to me.

OstreeFetcherPendingURI no longer needs its own reference count.

gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,leandrosansilva/ostree,leandrosansilva/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,leandrosansilva/ostree,leandrosansilva/ostree,leandrosansilva/ostree,gatispaeglis/ostree,gatispaeglis/ostree