db: shared compaction concurrency limit across multiple Pebble instances

In CockroachDB multi-store deployments, especially with large numbers of stores (8 or more), the CPU consumed by compactions can be significant. We do need a per store compaction concurrency limit, since disk bandwidth is a per-store resource, but we should additionally have a shared compaction concurrency limiter.

This shared limiter should fairly adjudicate on which compaction gets to run next based on a score of how important it is. For example,
- level score driven compactions could be compared based on the score, or additionally take into account the number of sub-levels if this is a compaction out of L0.
- a read-driven compaction in one store is strictly less important than a score driven compaction in another store.
- delete-only and move compactions are cheap, so don't need to respect this concurrency limit.

Jira issue: PEBBLE-230

Epic CRDB-41111