hammer2 - Refactor frontend part 11/many

hammer2 - Refactor frontend part 11/many

* Add XOPs for nresolve and most inode creation.  The inode creation support
  backs all inode-creation XOPs.

* Remove the on-stack hammer2_trans_t structure and remove the passing
  of &trans to all modifying operations.

  This methodology added significant list-management overhead and isn't
  a good fit with the new frontend/backend mechanics.  The removal also
  simplifies the API for many support functions.

  Replace with a temporary version that embeds a single, simplified
  hammer2_trans_t in each hammer2_pfs_t.  Ultimately we will need
  to fan-out to per-node (per hammer2_mount_t) transaction control
  in a way that doesn't stall-out the frontend.