Cluster: support host names when available, via CLUSTER SLOTS

Situation:

• especially from v7, cluster shards may be routed via hosts instead of IPs
• thus, the advertised IPs may sometimes (with known examples) may not be routable
• SLOTS and SHARDS (v7+) provide the hosts; NODES does not, as far as known

Proposal:

• prefer SLOTS to NODES (but not SHARDS due to v7 dependency)
• route via hostname when available, IP otherwise
• possibly with option to use fallback old behaviour?

Was: "Lookup by endpoint should check IP and host"

Currently, only exact endpoint matches are considered. However, specially in the case of CLUSTER, a node may have both host and IP evidently. Further, -MOVED may return the unexpected option, leading to additional connections.

We should:

• record both identities as declared, for example from the CLUSTER NODES response
• if no equality endpoint match is found, check also using the best data available (after type-testing the endpoint)

Context: https://github.com/dotnet/aspnetcore/issues/59211

Auxiliary consideration: should cluster routing be limited to IP, as current? Should there be a "use host routing" option? Or should we try IP and use host if that fails?