Check if connection is excess in connection_dir_finished_flushing

In legacy/trac#4483 (moved), I added a check like the one below to every transition from DIR_CONN_STATE_CONNECTING to another state, but I missed connection_dir_finished_flushing:

        /* Close this connection if there's another consensus connection
         * downloading (during bootstrap), or connecting (after bootstrap). */
        if (connection_dir_close_consensus_conn_if_extra(conn)) {
          return;
        }

It would be good to to fix this in 0.2.8, because otherwise clients could end up requesting multiple consensuses, which is expensive for relays and for the network.