hammer2 - More involved refactoring of chain_repparent, cleanup

hammer2 - More involved refactoring of chain_repparent, cleanup

* Remove unused locking flags (remove the NOLOCK and NOUNLOCK
  features).

* Add HAMMER2_RESOLVE_NONBLOCK to hammer2_chain_lock() for use
  only by hammer2_chain_getparent() and hammer2_chain_repparent().

* Refactor hammer2_chain_getparent() and hammer2_chain_repparent().
  Add a hot-path that uses HAMMER2_RESOLVE_NONBLOCK.  If this fails
  we now do a much more involved tracking operation via 'reptrack'
  to deal with races against indirect block deletions.

* Cleanup the copyright messages.

* Fix an issue where a sync could be held-up indefinitely by
  ongoing overlapping modifying operations.

* Install a proper initial inode count when creating a snapshot.

* Fix a deadlock in checkdirempty().  A chain lock was winding
  up being ordered incorrectly.