## Problem
When image compaction yields for L0 compaction, it may not immediately
schedule L0 compaction, because it just goes on to compact the next
pending timeline.
Touches #10694.
Requires #10744.
## Summary of changes
Extend `CompactionOutcome` with `YieldForL0` and `Skipped` variants, and
immediately schedule an L0 compaction pass in the `YieldForL0` case.