Investigate using a single AppStream pool

Currently, we use an AsPool instance per backend (PackageKit/Flatpak) mostly as a way of easily determining which backend should handle a given component.

The original intent of the AppStream API was for software centers to use a single pool for all components and in theory, there should be a great number of simplifications/optimisations we gain from doing so.

I seem to remember I found some blockers when I investigated that route in the past, then looked at other software centers and found they were using multiple pools, so just went that route and didn't think much more about it.

This is mostly a note to future me, or anyone else working on AppCenter that this seems very much worth investigating and any feedback we can provide to AppStream on making this work better would be much appreciated upstream.

Related discussion:
https://github.com/ximion/appstream/issues/269