added missing methods (stubs) required for snitch implementation

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>

New in v2:
   - storage_service: add a non-const version of get_token_metadata().
   - get_broadcast_address(): check if net::get_messaging_service().local_is_initialized()
     before calling net::get_local_messaging_service().listen_address().
   - get_broadcast_address(): return an inet_address by value.
   - system_keyspace: introduce db::system_keyspace::endpoint_dc_rack
   - fb_utilities: use listen_address as broadcast_address for now

locator/token_metadata.hh

@@ -1072,6 +1072,11 @@ public:
         }
     }
 #endif
+    void invalidate_cached_rings() {
+        // TODO:
+        //ringVersion++;
+        //cachedTokenMap.set(null);
+    }
 };
 
 }