v5.9.0

Added

• (version) keep only the current version (#643)
• (config) change compression policies on a live tree (#642)
• (compression) let the tree own its zstd dictionaries (#640)

Documentation

• (compaction) write down the two couplings the folds rest on (#639)

Fixed

• (compaction) refuse an interrupted merge, and record only real losses (#636)
• (tree) record the GC a flush performed, and only what a run collected (#635)
• (compaction) keep the newest version below the GC threshold (#631)

Refactored

• (compaction) name the GC watermark once (#644)