## Problem
`CompactFlags::NoYield` was a bit inconvenient, since every caller
except for the background compaction loop should generally set it (e.g.
HTTP API calls, tests, etc). It was also inconsistent with
`CompactionOutcome::YieldForL0`.
## Summary of changes
Invert `CompactFlags::NoYield` as `CompactFlags::YieldForL0`. There
should be no behavioral changes.