zones/records not synced after re-adding dns-backend relation

Existing zones/recordsets are not synced when the relation dns-backend is added (e.g. adding a relation to designate-bind)

# steps to reproduce
1 - Deploy a bundle with designate/designate-bind (e.g. zaza test bundle, I used bionic-queens)
2 - Create a zone and an associated recordset
3 - Remove the dns-backend relation
4 - Add a new designate-bind application with a different name + associated unit(s)
5 - Add the dns-backend relation between designate and the new designate-bind application from the previous step.
6 - List the zone/records from step 2, they will be reported a status: Active, action: None, but the zone/records will not be present in the designate-bind unit (e.g. nothing in /var/cache/bind)
7 - Attempt to resolve the dns records from the second application, it will fail.

Adding a new record to an existing zone will sync it completely, so pre-existing records will work. However, it would be good to at least have an action to sync all zones to make sure the dns server syncs quickly after switching over to new units.