fetcher: Track outstanding requests with a table

fetcher: Track outstanding requests with a table

Track outstanding HTTP requests in a table for easier debugging.

Also fixes a bug discussed in https://bugzilla.gnome.org/755224
where the outstanding request counter was not decremented in the
event of an error, which could result in the fetcher hitting its
max request limit and locking up.

The bug is fixed by removing the request struct from the table in
pending_uri_free(), which is always called regardless of error,
so the outstanding request count is always accurate.