Plan, test and document 3.0 migration path for distros

When Tracker 3.0 is released we to tell distros how migrate from 2.3.

Tracker has a small number of known users. It's possible we could update Grilo, Nautilus and the gnome-* apps to use Tracker 3.0 in the same GNOME cycle that Tracker 3.0 is released. If not, we will need to test and support parallel-installed 2.3 and 3.0.

Flatpak apps will need to be updated. From the OS perspective, new apps will communicate via xdg-tracker-portal while old apps will expect to talk to org.freedesktop.Tracker1.* on DBus. Do new distros need to support old apps? (and do new apps need to support old distros?)

Users will have data in ~/.cache/tracker. This could include 22GB of journal files. Can some automated cleanup be done?