Start health checks of all running app versions after leader change

Before only the current version's health checks were started on leader change,
leading to missing health checks when, e.g. after scaling, when old app versions
were still running.

Fixes #912
It appears that here, the health check manager will look up the app by id, finding the current version instead of the updated version.  Either the update should be written through to the repo before calling `reconcileWith`, or some other workaround is needed.