support allow_ingest_behind with level compaction

[ajkr]

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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!