pcr: no visibility into / reported progress during initialization

I initialized replication from one 35TB cluster to another. The initialization process is taking several days, but during this time is giving no indication of when it will be caught up and ready for a potential cutover; I cannot tell if I'll have a ready standby in minutes, hours or days, or if it is even still making progress towards being ready at all, or has become stuck and I need to intervene.

While we don't have a clear concept of "progress" during steady-state replication, we do during initial scan, when we're going from no data replicated to all data replicated to provide cutover to at least the start time, so we should show % of ranges / spans that have advanced to the start time or above.

Jira issue: CRDB-36716