kvserver: pace range split stats computation

[kvoli]

Is your feature request related to a problem? Please describe.

After #22348, the amount of data scanned for stats computation increased. When there are multiple splits occurring in parallel on the same store, this can consume a large amount of CPU.

Describe the solution you'd like

Pace range split stats computation, such as integrating with the elastic AC CPU limiter.

Describe alternatives you've considered
We should also spread out splits where possible, to reduce the likelihood of multiple splits occurring at once. See #22348.

Additional context

Note that some range splits, such as load based, should not be delayed as timely splits are depended upon for load rebalancing. Similarly, a range will begin backpressuring writes after the range exceeds a certain threshold above the maximum range size -- any solution here will need to make a tradeoff between backpressure and pacing stats computation.

Jira issue: CRDB-39679

Epic CRDB-39952