bgpd: fix bgp_node locking issues

bgpd: fix bgp_node locking issues

* bgpd: Connected table locks were being locked but not unlocked, such that
  eventually a lock would exceed 2^31 and become negative, thus triggering
  an assert later on.
* bgp_main.c: (bgp_exit) delete connected elements along with ifp's.
* bgp_nexthop.c: (bgp_nexthop_lookup{,_ipv6}) add missing unlocks
  (bgp_multiaccess_check_v4) ditto
  (bgp_connected_{add,delete}) Use a distinct memtype for bgp_connected_ref.
  (bgp_scan_finish) reset the nexthop cache to clean it up when bgpd exits
* bgp_route.c: fix missing bgp_node unlocks
* lib/memtype.c: (memory_list_bgp) add MTYPE_BGP_CONN
* testing: has been tested for almost 2 months now.