kvserver: lease transfers resulting from preference violations are unpaced

**Describe the problem**

We purposefully do not pace lease transfers when they are the result of preference violations. This makes sense in many cases, but when there are many thousands of leases to move, this can cause a flurry of lease transfers, impacting tail latencies as a request hits many NLHEs at once.

One solution here may be to provide an opt-in delay for lease-preference induced transfers.

Jira issue: CRDB-66686

Start with the issue description and Jira issue CRDB-66686, then trace the existing pacing behavior for lease transfers caused by preference violations. Clarify the opt-in delay design and define done as preventing transfer bursts while preserving the intended preference-repair behavior.

Written by the indexing model from the issue text.