support `allow_ingest_behind` with level compaction

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### Expected behavior

When `allow_ingest_behind` is set, automatic level compactions should not write output files to the bottom level (`ColumnFamilyOptions::num_levels - 1`). That way, a sorted run of files can be ingested behind without conflict.

### Actual behavior

Files are written to the bottom level by level compaction, which can cause overlap even for ingest-behind of non-overlapping files: `Invalid argument: Can't ingest_behind file as it doesn't fit at the bottommost level!`