Ensure that calling Next twice in a row without first calling Reset is
safe when the iterator is at the end of its list. Also alter the
towerListIterator to call Reset after hitting an error on Next.
This commit adds a test that shows that it is possible to cause the
AddressIterator to panic if the `Next` method is ever called twice when
the iterator is at the end of its list without Reset first being called.
In this commit, a new AddressIterator type is added. It is a type that
can be used to iterate over a list of addresses. It strictly disallows
the list of addresses it holds to be empty. It also allows callers to
place locks on certain addresses in order to prevent other callers from
removing the addresses in question from the iterator.
