Leverage cluster size hint for service readiness

[MirahImage]

Is your feature request related to a problem? Please describe.
When a cluster is forming, the nodes become available to the service immediately when they come online. If a quorum queue is created early on when only some of the nodes are online, then the quorum queue will not be spread across a full set of nodes, and each node will be quorum critical. This can block cluster upgrades, and requires manual intervention to add the remaining node(s) to the quorum set.

Describe the solution you'd like
We set cluster_formation.target_cluster_size_hint = ${number-of-replicas}. If we can leverage this to ensure the cluster is not available via the service until the condition is met (or failing that, ensure that RabbitMQ itself does not accept connections until the condition is met), it would prevent this situation

[michaelklishin]

RabbitMQ itself currently imports definitions only when the current number of cluster members matches the hint (which defaults to 1).

Client connection listener is already basically the last step during boot. Refusing connections based on what cluster nodes have joined sounds pretty radical to me, and can make edge cases such as the famous K8S-specific deadlock more likely.