cluster: fence stale host events by endpoint

Track the endpoint tied to host up/down handling, reconnection callbacks, and pool cleanup so stale work from a previous endpoint cannot mark or reconnect a replacement host.

Preserve endpoint-specific identity for SNI and client-routes endpoints, scope non-retryable auth failures to the matching endpoint, and remove stale pools by host identity instead of endpoint equality.

Add unit coverage for endpoint swaps, queued up/down races, stale reconnector success, and defunct connection handling after client-route port changes.

Author: dkropachev