Make 3.9 fifo_client forward-compatible with 3.10

Currently, when upgrading to 3.10, some errors may occur that
are fairly innocent - the client should just reconnect to another node
and everything should be back to normal. This PR will prevent errors for
those who will upgrade to 3.10 from more recent 3.9 patch releases.

Example error:
```
** {{case_clause,2},
    [{rabbit_fifo_client,enqueue,3,
                         [{file,"rabbit_fifo_client.erl"},{line,150}]},
     {rabbit_quorum_queue,'-deliver/2-fun-0-',4,
                          [{file,"rabbit_quorum_queue.erl"},{line,851}]},
     {lists,foldl,3,[{file,"lists.erl"},{line,1267}]},
     {rabbit_queue_type,'-deliver0/3-fun-3-',4,
                        [{file,"rabbit_queue_type.erl"},{line,480}]},
     {maps,fold_1,3,[{file,"maps.erl"},{line,410}]},
     {rabbit_queue_type,deliver0,3,
                        [{file,"rabbit_queue_type.erl"},{line,479}]},
     {rabbit_queue_type,deliver,3,[{file,"rabbit_queue_type.erl"},{line,454}]},
     {rabbit_channel,deliver_to_queues,2,
                     [{file,"rabbit_channel.erl"},{line,2190}]}]}
```

Author: mkuratczyk

deps/rabbit/src/rabbit_fifo_client.erl

@@ -151,7 +151,7 @@ enqueue(Correlation, Msg,
         0 ->
             %% the leader is running the old version
             enqueue(Correlation, Msg, State0#state{queue_status = go});
-        1 ->
+        N when is_integer(N) ->
             %% were running the new version on the leader do sync initialisation
             %% of enqueuer session
             Reg = rabbit_fifo:make_register_enqueuer(self()),