[packagekit] Helpful functionality for threaded backends

[Richard Hughes]

I've added some new functionality you might find useful if you
maintain a threaded backend:

commit eb00ffb63672bb35dd4ad6ca486fb6e39b61c85c
Author: Richard Hughes <richard at hughsie.com>
Date:   Tue Sep 14 15:14:27 2010 +0100

    Add transaction_start() and transaction_stop() vfuncs to make
backends simpler

    For threaded backends, there is a lot of copy and paste code in
    just setting up and tearing down the packaging system for each
    transaction. By making this common code we can make our lives
    much easier.

    The new vfuncs are optional, and nothing uses them yet.

You probably want to "git show
eb00ffb63672bb35dd4ad6ca486fb6e39b61c85c" and read the comments. I'm
going to switch the yum backend to it and see how many lines of code I
can remove.

Richard.