MXS-936: add shutdown hooks

First part: add shutdown hooks for routers.

Binlog is the first one with a  destroyInstance() routine

server/modules/routing/schemarouter/schemarouter.c

@@ -113,7 +113,8 @@ static ROUTER_OBJECT MyObject =
     diagnostic,
     clientReply,
     handleError,
-    getCapabilities
+    getCapabilities,
+    NULL
 };
 
 static bool rses_begin_locked_router_action(ROUTER_CLIENT_SES* rses);