hammer2 - flush sequencing part 6 - stabilization pass

hammer2 - flush sequencing part 6 - stabilization pass

* Fix bug when chains are e.g. delete/duplicated where a flush can occur
  out of order and retire more recent chain structures leaving older
  CHAIN_DELETED structures behind.  This caused the merged chain_find
  and lookup/iteration code to improperly believe a slot was empty when
  in fact the parent's bref array had a valid blockref.

  Fixed by flagging the discrete chain->next_parent linkages with
  CHAIN_IPACTIVE and adding a ref to the chain, which prevents them
  from being retired out of order.  This is currently being done
  unconditionally (and probably has to be done unconditionally).

* Fix bug related to the clearing of SUBMODIFIED during a flush.  Generally
  speaking a flush pass clears SUBMODIFIED if no children were deferred.

  The bug is that higher-level indirect-block chains closer to the root
  are typically only modified during the flush as the deeper chains are
  flushed recursively and rolled up into the parents on the way back up.
  Because of this, these higher-level chains could represent more than one
  flush synchronization point.  When this situation is detected we now
  set SUBMODIFIED here too.

* Consolidate ip->chain adjustments into the hammer2_inode_repoint()
  procedure.  Remove hammer2_inode_put().

* Cleanup some #if 0'd code that I won't be using.