* Use weakref's of DBCursor objects for the iterator cursors to avoid a

* Use weakref's of DBCursor objects for the iterator cursors to avoid a
  memory leak that would've occurred for all iterators that were
  destroyed before having iterated until they raised StopIteration.

* Simplify some code.

* Add new test cases to check for the memleak and ensure that mixing
  iteration with modification of the values for existing keys works.