Bulk load mode for LCS

We can currently avoid LCS write amplification on bulk load as follows:

1. Disable automatic compaction
2. Load all the data
3. Force major compaction
4. Re-enable automatic compaction

It would be nice to automate this. E.g. if we detect that L0 contains as much data as the rest of our arena, skip bubbling sstables up and just major compact automatically. In a bulk load situation, this would result in exponentially increasing major compaction sizes – not as ideal as the manual form with just a single one, but much better than our current behavior which is not a good experience for users.