fix: get next_sstable_id

[jobala]

Summary

• checks through all versions to get the next sstable id.

closes #541

[ethe]

The fix only scans all versions when latest_version is None or has empty SSTs. If latest version has SSTs, it only uses those. This works under the assumption that SST IDs are always monotonically increasing, so the latest version's SSTs will always have IDs >= any older version's SSTs. This assumption holds for normal operation. This fits the expectation of #541, thank you @jobala !

Please consider adding a regression test to tests/ directory that covers this specific scenario to prevent future regressions.

[jobala]

Thanks for reviewing @ethe, I have added tests.

[ethe]

Looks good to me! Thank you @jobala