HAMMER 59A/Many: Mirroring related work (and one bug fix).

HAMMER 59A/Many: Mirroring related work (and one bug fix).

* BUG FIX: Fix a bug in directory hashkey generation.  The iterator could
  sometimes conflict with a key already on-disk and interfere with a pending
  deletion.  The chance of this occuring was miniscule but not 0.  Now fixed.

  The fix also revamps the directory iterator code, moving it all to one
  place and removing it from two other places.

* PRUNING CHANGE: The pruning code no longer shifts the create_tid and
  delete_tid of adjacent records to fill gaps.  This means that historical
  queries must either use snapshot softlinks or use a fine-grained
  transaction id greater then the most recent snapshot softlink.

  fine-grained historical access still works up to the first snapshot
  softlink.

* Clean up the cursor code responsible for acquiring the parent node.

* Add the core mirror ioctl read/write infrastructure.  This work is still
  in progress.

  - ioctl commands
  - pseudofs enhancements, including st_dev munging.
  - mount options
  - transaction id and object id conflictless allocation
  - initial mirror_tid recursion up the B-Tree (not finished)
  - B-Tree mirror scan optimizations to skip sub-hierarchies that do not
    need to be scanned (requires mirror_tid recursion to be 100% working).