Sometimes we have a link which is no longer in use within our
infostructure.  However, it's not a good idea to throw away
information about it immediately.  It could be that certain files have
been temporarily deleted and will come back.  Alternatively, a link
could have been found broken and been corrected, but someone has a
copy of the old page.  When they reinstall the copy, we will have to
deal with that link again.  

If we had not kept that link around and they immediately do a link
check on their document they may see nothing wrong, and, because of
the nature of link-controller, we won't start reporting the link as
broken until we have confirmed that it really is, some days later.

If, on the other hand, we keep checking all of the links which are
aged, we will cause ourselves considerable extra work.  

To handle this, links are aged.  Once a link has reached greater than
a certain age, the link will not be checked any more.   Once the link
has reached a much larger age, it will be completely deleted from the
link database.  The age is reset each time the link is extracted from
the infostructure, so links which are still in use will continue to be
checked.  

In the meantime, the link will still be repeatedly scheduled based to
it's normal checking time.  This causes us to examine it quite
regularly, but that is okay, since we will do almost no work when we
look at it.

By default links which have not been refreshed will be ignored after
one week and will be deleted from the database after two months.  
