This class holds information about configuration elements which have
changed, if the changed element is likely to affect the way the zone is
audited. It keeps a list of the elements which we are interested in, the
last value they were seen to have, and the timestamp at which that value
changed (0 if they have not been seen to change). It allows the auditor to
respond to changes in policy, and not raise errors when it sees records in
the zone which still conform to the old policy. @TODO@ TEST CODE FOR THIS
CLASS! @TODO@ TEST CODE FOR THE POLICY CHANGES FUNCTIONALITY - check that
the auditor
does not produce errors as a result of changes in policy.