ospfd: Optimize and improve SPF nexthop calculation

ospfd: Optimize and improve SPF nexthop calculation

Maintain router LSA positions in OSPF interface.
Find the OSPF interface in nexthop_calculation using
the position in the router LSA. This is possible because
the only time nexthop_calculation needs to look up interfaces
is when dealing with its own Router LSA.

This has the following advantages:
 - Multiple PtP interfaces with the same IP address between two routers.
 - Use Unnumbered PtP on just one end of the link.
 - Faster OI lookup for the OSPF interface and only
   done once for PtoP links.

*ospf_interface.h: (struct ospf_interface) Add storage for
		   storing router LSA position.

*ospf_interface.c: (ospf_if_lookup_by_lsa_pos)
		   lookup OSPF I/F in an area using LSA position.

*ospf_lsa.c: (router_lsa_link_set) record Router LSA position.

*ospf_spf.c: (ospf_spf_next) Count and pass along lsa position.
	     (ospf_nexthop_calculation) Add lsa position argument.
	     call ospf_if_lookup_by_lsa_pos() for OSFP interface handle.
	     Clean up and remove all calls ospf_if_is_configured() the
	     rest. Adjust a few debug logs.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>