Make undo{Log,Info} arguments act like Python slice indices when
both are non-negative.  The code used to do that before ZODB 3.4a9,
but changed to match ZODB's UML documentation.  Alas, some
(untested) code in Zope relied on the actual behavior (see the
collector report).  Changed code, docs, and tests to bless the
old behavior in these cases.

DemoStorage.UndoLog:  this was wrong in several ways.  I'm still
unsure about why it skips "packed" transactions.  That doesn't seem
right, but I don't have time to wonder about that now.