MXS-2350: Connect on session command with lazy_connect

If a session command is executed when lazy_connect is enabled and no
connections have been created, a connection must be made. This makes sure
that the session isn't closed and that the client receives a response.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -163,6 +163,7 @@ private:
     bool            handle_got_target(GWBUF* querybuf, mxs::RWBackend* target, bool store);
     void            handle_connection_keepalive(mxs::RWBackend* target);
     bool            prepare_target(mxs::RWBackend* target, route_target_t route_target);
+    bool            create_one_connection();
     void            retry_query(GWBUF* querybuf, int delay = 1);
 
     bool should_replace_master(mxs::RWBackend* target);