HAMMER - Fix cursor tracking bugs and a few other issues

HAMMER - Fix cursor tracking bugs and a few other issues

* When recursively removing empty internal nodes from the B-Tree only
  call hammer_cursor_deleted_element() if the related internal
  element is actually removed.  The element might not be removed due
  to the deadlock fail path.

* If hammer_cursor_up_locked() fails fully restore the cursor before
  returning.  The index field was not being restored.

* Acquire the sync lock when recovering a cursor lost due to a deadlock
  in the mirroring code.

* Document and fix an issue in the rebalancing code which could cause a
  cursor to fall off the end of the B-Tree.

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