pk: Load the AppStream database on a separate thread
Summary:
No need to block the backends load on PackageKit appstream usage.
BUG: 409262
BUG: 409599
FIXED-IN: 5.18.0
Test Plan: Tested manually
Reviewers: Discover Software Store, broulik, Plasma
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D26547