fetcher: Fix previous commit

I was getting hangs in the test suite, and looking at the previous
commit, we were calling the async completion functions out of the
finalizer for the URI, which is weird.  I didn't analyze what's going
wrong, but what we really should be doing is processing our internal
queue after we've downloaded a file, and the request is about to be
finalized.

I suspect doing queue management from the finalizer created a circular
reference type situation.

This patch deduplicates the queue processing bits too.

https://bugzilla.gnome.org/show_bug.cgi?id=708126