got registered. (Deja vu all over again. ;)

Changed to commit objects in registration order and to allow
additional objects to get registered while commiting. (The
later was wanted by PJE.)

Added a comment in commit to explain the "invariant".

Fixed bug that caused objects to get multiply aborted.  Now always
clear object registry on commit and abort. This seems like a good
idea anyway. (We were relying on free_transaction to have the
same effect. This faild miserably in single-threaded apps and
in certain error conditions.

Added a teeny optimization in get_transaction().