Comment 0 for bug 1831981

[Impact]
aptdaemon needs to implement frontend locking to avoid it stumbling over clients that do implement it. We also change the locking order to lock in APT order, and the unlock order becomes the reverse of that so that the behavior is sane wrt other clients.

[Test case]
N/A

[Regression potential]
There may be other race conditions appearing now due to the order of (un)locking changing.