OSPF LSA may not be refreshed within 1800s, ANVL OSPF test 12.4 fails

OSPF LSA may not be refreshed within 1800s, ANVL OSPF test 12.4 fails

Signed-off-by: Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-10948

The previous formula for calculating delay was:

    delay = OSPF_LS_REFRESH_TIME - LS_AGE (lsa) - OSPF_LS_REFRESH_JITTER
              + (random () % (2*OSPF_LS_REFRESH_JITTER));

If LS_AGE() is 0 we get:

  delay = 1800 - 0 - 60 + (random() % 2*60)

So delay will be 1740 + 0..120, so we could easily pick a delay that is
greater than OSPF_LS_REFRESH_TIME (1800).  For the normal case where
we hit this else the LS_AGE will be > 0 so that reduces the chances of
delay being greater than 1800.