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
diff --git a/ospfd/ospf_lsa.h b/ospfd/ospf_lsa.h
index 8dd054c..251d473 100644
--- a/ospfd/ospf_lsa.h
+++ b/ospfd/ospf_lsa.h
@@ -80,6 +80,7 @@
 #define OSPF_LSA_DISCARD	  0x10
 #define OSPF_LSA_LOCAL_XLT	  0x20
 #define OSPF_LSA_PREMATURE_AGE	  0x40
+#define OSPF_LSA_IN_MAXAGE	  0x80
 
   /* LSA data. */
   struct lsa_header *data;