Switch to using a peer relation for application data sharing

ops-lib-pgsql v1 relies on leader-set/leader-get to share information between peers (because Application non-peer relation data is not readable by non-lead units of the Application, and they have no other way to know what choices the leader made). Unless this gets fixed in Juju, we should consider requiring client charms to declare a peer relation and use Application relation data on that (because Application peer relation data *is* readable by non-lead units of the Application, which is *not at all* confusing to new charmers). The main reason for this change is that leadership is not exposed by the Operator Framework, and the wrappers ops-lib-pgsql uses instead are not tied in to the Operator Framework testing harness.

Alternatively, if https://github.com/canonical/operator/issues/215 gets addressed and leader-get/leader-set not deemed deprecated, then we can not make this change and non increase the burden on ops-lib-pgsql users.