db: consider splitting ingested sstables

[jbowens]

While skimming #517, I saw @sumeerbhola's comment:

Should we provide split points to the ingestion logic to ensure it does not produce wide sstables?

Once we have virtual SSTs, we could ingest sstables as virtual SSTs, split according to existing heuristics including L0 flush splits.

Do we ever see imports where broad ingested sstables restrict compaction concurrency out of L0? I would guess not, since Cockroach only ingests sstables as broad as a single KV range.

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it
in 10 days to keep the issue queue tidy. Thank you for your
contribution to Pebble!