[packagekit] pk_backend_finished race issue

[Tom Parker]

Richard Hughes wrote:
> I've pushed all my threading changes to the server (which is back up
> now) and I've probably broken some backends. Please pull, fix and test
> and hopefully everything should work 100%. I'll push to anon fd.o
> tonight.

Confirm that apt backend works fine now (once I fixed a nasty little bug 
in pk_backend_thread_helper), and I've committed the changes necessary 
for it to be ok with new interfaces again.

Tom