HAMMER 34/many: Stabilization pass

HAMMER 34/many: Stabilization pass

* Fix a node reference count leak in hammer_btree_lock_children().  This
  was resulting in a panic at umount time.

* Fix a misordered lock in hammer_cursor_up() which could result in
  an assertion in the B-Tree iteration code.  The cursor-up code was
  resolving the parent index prior to acquiring a shared lock on the
  parent node.  An insertion by another thread could do a split and
  change the parent index.  The shared lock must be acquired first.

* Add additional debugging output and add additional assertions.  HAMMER
  now has 205 assert lines in the code.

Reported-by: YONETANI Tomokazu <qhwt+dfly@les.ath.cx> (umount panic)