HAMMER 38E/Many: Undo/Synchronization and crash recovery

HAMMER 38E/Many: Undo/Synchronization and crash recovery

* Clean up interlocks between the frontend and backend.

* Deal with the case where the backend needs to sync a record to disk that
  the frontend wishes to delete.  This basically just involves converting
  the record from a deleted in-memory record to a delete-on-disk record,
  so the synced record does not become visible to userland.

* Deal with the case when an inode is being destroyed where the backend
  wishes to delete an in-memory record without syncing it to disk.

* Document numerous special cases for future attention.