Fix memory corruption: add book_closing and use it.

[Derek Atkins]

Log Message:
-----------
Fix memory corruption: add book_closing and use it.

	* src/engine/Transaction.c:
	  Don't recompute balances or write to the translog when we're
	  shutting down.  Destroy the parent transaction from xaccSplitDestroy()
	  if we're shutting down.
	* src/engine/qofbook-p.h:
	* src/engine/qofbook.h:
	* src/engine/qofbook.c:
	  add "shutting_down" parameter and getter-method, so that
	  objects can detect when the book is shutting down and
	  ignore non-necessary reprocessing.  Fixes a memory corruption
	  bug during book-closing.

Modified Files:
--------------
    gnucash:
        ChangeLog
    gnucash/src/engine:
        Transaction.c
        qofbook-p.h
        qofbook.c
        qofbook.h