fetcher: Clean up code to avoid intermediate files

I think originally we had the .part/.done separation because we were
trying to support partial downloads of files like repo/config and
repo/refs.

But now that the http server configuration won't give us partial
results, we don't need to support caching those files between runs.

And thus, there's no reason to have the .part/.done and do the dance
with renaming them.

When fetching objects/ and other things that use _with_async, we
continue to use _append_to(), and if the returned range tells us we
have all the bytes, then we hand the full file over to the caller.

Don't attempt to shortcut in the case where the last run told us we
already have the object; the object fetcher code will not make a
request.

While we're here, also clean up use of GError and consistently use the
cancellable from the pending.

https://bugzilla.gnome.org/show_bug.cgi?id=707157
fetcher: Only open files when we are ready to write to them

Otherwise we quickly run out of file descriptors when doing large
requests.

https://bugzilla.gnome.org/show_bug.cgi?id=707157
libostree: Change synchronous fetching API to return a stream

There's not a good reason to write small things such as repo/config to
the filesystem, only to read them back in again.  Change the
non-partial API to just return a stream, then read it into a memory
buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=707157