The recent change to the charm to gate sending connection data to
clients until the internal clustering is complete introduced a code
path that exists amqp_changed without sending connection data to
clients. But update_clients assumes that the client update always
happens and sets INITIAL_CLIENT_UPDATE_KEY to refelct that. This
change updates the logic to only set INITIAL_CLIENT_UPDATE_KEY when
the clients have actually been updated.
The previous change to rabbit also added calls to update_clients
from leader settings changed and cluster changed hooks. This change
also adds a small optimisation to gate those new calls on whether
the charm has sufficient peers.
