Handle partial metadata sync scenario

Hi,

We have this scenario where the leader unit successfully synced and verified a snapshot of the archive metadata, it syncs to it's peers but is then interrupted. On next sync, we see this:

| 2023-06-15 00:10:34,973 - Syncing /srv/ubuntu-repository-cache/apache/data/ubuntu_2023-06-14_23:04:01_u2/ to ubuntu-repository-cache/1 (192.168.0.5) www-sync@192.168.0.5:/srv/ubuntu-repository-cache/apache/data/ubuntu_2023-06-15_00:04:01_u2.
| 2023-06-15 00:10:34,973 - timeout 3600 /usr/bin/rsync -r --quiet --recursive --links --perms --chmod=g+w --times --compress --delete --delete-before --timeout=300 -e sudo -u www-sync ssh -l www-sync --link-dest=/srv/ubuntu-repository-cache/apache/data/ubuntu_active/ /srv/ubuntu-repository-cache/apache/data/ubuntu_2023-06-14_23:04:01_u2/ www-sync@192.168.0.5:/srv/ubuntu-repository-cache/apache/data/ubuntu_2023-06-15_00:04:01_u2

From the above, `ubuntu_active` is still pointing at `ubuntu_2023-06-14_23:04:01_u2` but on pushing to it's peers, it's pushed to `ubuntu_2023-06-15_00:04:01_u2` - that's 00:04 vs. 23:04. Then the `ubuntu_active` symlink is synced across and that points to an invalid or non-existent metadata snapshot.

Full logs - https://pastebin.canonical.com/p/rNNNKvs9mN/ and https://pastebin.canonical.com/p/mnv9xnztfP/

We should handle this by making sure the synced destination matches the source (or timestamp/meta_ver `ubuntu_active` currently points to).