HAMMER 44/Many: Stabilization pass, user-guaranteed transaction ids

HAMMER 44/Many: Stabilization pass, user-guaranteed transaction ids

* B-Tree changes:  Allow leaves to be empty.  Do not leave internal nodes
  with subtree_offsets of 0 when deleting a B-Tree element.  Do not try to
  clean up internal nodes with subtree_offsets of 0 while scanning the B-Tree.

  The pruner will be made responsible for such cleanups.  This way the
  front-end does not modify the B-Tree at all.

* Add a new ioctl to support the hammer utility 'synctid' command.