ospfd: fix performance problem with redistribute delete

ospfd: fix performance problem with redistribute delete

Doing redistribute delete with full BGP table was taking
30 minutes, this drops it down to less than a second.

* ospf_lsa.c: (ospf_lsa_maxage) When flushing lots of entries the
  performance is terrible because it looks up each LSA entry through
  ospf_lsa_maxage_exist before deleting causing O(N^2) performance.  Use a
  new OSPF_LSA_MAXAGE flag instead of scan - and maintain it.
  (ospf_lsa_maxage_exist) removed
  (ospf_lsa_maxage_delete) maintain OSPF_LSA_MAXAGE flag