hammer2 - Major restructuring, part 4/several

* Add inumber -> inode structure tracking and lookup.  This is needed to
  ensure that only a single inode structure be used to track multiple
  hardlinks to the same place.

* Continue stabilization.  Remove modify_tid/delete_tid checks in the
  flush code and (for now) only flush along the live path.

  Refactor held chains when creating new chains.  The creation of a new
  chain can move around existing chains, causing the held chain to be
  marked deleted.

  When a hardlink is consolidated in a parent directory the source chain
  used in the duplication is not deleted.  Numerous chain->duplink
  handling code was assuming that the source was always deleted.  Fix
  that.

* The shared inode lock now refactors ip->chain (the exclusive inode lock
  already did so).

* Fix most ref-counting of the chain structure, fixing most of the memory
  leakage issues on unmount.

* There are still some issues with small files not inheriting their data
  on duplication.  cpdup /usr/share works but a significant number of
  small files lose their data references on re-mount.
hammer2 - Improve 'hammer2 show'

* The show directive now chooses the best volume header by default
  (but will dump all four if you specify -vvv).