v0.5.4

Removed

• Removed the external-log state-machine surface from holt core:
  Durability::StateMachine, DB::commit_durable,
  Tree::commit_durable, durable_applied_index, DB::scatter,
  DB::scatter_independent, and the file-store DurableManifest
  trailer.
• Checkpoint images are now pure DB archive/transfer images. They
  contain family key/value data and no longer carry an external
  applied_index.
• Atomic DB/Tree batches always use the exclusive mutation gate again;
  holt no longer has a StateMachine-only relaxed batch mode.