rabbitmq_peer_discovery_consul: Select the node to join

[Why]
The default node selection of the peer discovery subsystem doesn't work
well with Consul. The reason is that that selection is based on the
nodes' uptime. However, the node with the highest uptime may not be the
first to register in Consul.

When this happens, the node that registered first will only discover
itself and boot as a standalone node. Then, the node with the highest
uptime will discover both of them, but will select itself as the node to
join because of its uptime. In the end, we end up with two clusters
instead of one.

[How]
We use the `CreateIndex` property in the Consul response to sort
services. We then derive the name of the node to join after the service
that has the lower `CreateIndex`, meaning it was the first to register.

deps/rabbitmq_peer_discovery_consul/src/rabbit_peer_discovery_consul.erl

@@ -83,8 +83,9 @@ list_nodes() ->
                                                                     HttpOpts) of
                                    {ok, Nodes} ->
                                        IncludeWithWarnings = get_config_key(consul_include_nodes_with_warnings, M),
-                                       Result = extract_nodes(
-                                                  filter_nodes(Nodes, IncludeWithWarnings)),
+                                       Result = extract_node(
+                                                  sort_nodes(
+                                                    filter_nodes(Nodes, IncludeWithWarnings))),
                                        {ok, {Result, disc}};
                                    {error, _} = Error ->
                                        Error
@@ -276,13 +277,24 @@ filter_nodes(Nodes, Warn) ->
     false -> Nodes
   end.
 
--spec extract_nodes(ConsulResult :: [#{binary() => term()}]) -> list().
-extract_nodes(Data) -> extract_nodes(Data, []).
-
--spec extract_nodes(ConsulResult :: [#{binary() => term()}], Nodes :: list())
-    -> list().
-extract_nodes([], Nodes)    -> Nodes;
-extract_nodes([H | T], Nodes) ->
+-spec sort_nodes(ConsulResult :: [#{binary() => term()}]) -> [#{binary() => term()}].
+sort_nodes(Nodes) ->
+  lists:sort(
+    fun(NodeA, NodeB) ->
+        IndexA = maps:get(
+                   <<"CreateIndex">>,
+                   maps:get(<<"Service">>, NodeA, #{}), undefined),
+        IndexB = maps:get(
+                   <<"CreateIndex">>,
+                   maps:get(<<"Service">>, NodeB, #{}), undefined),
+        %% `undefined' is always greater than an integer, so we are fine here.
+        IndexA =< IndexB
+    end, Nodes).
+
+-spec extract_node(ConsulResult :: [#{binary() => term()}]) -> list().
+extract_node([]) ->
+    [];
+extract_node([H | _]) ->
   Service = maps:get(<<"Service">>, H),
   Meta = maps:get(<<"Meta">>, Service, #{}),
   NodeName = case Meta of
@@ -299,7 +311,7 @@ extract_nodes([H | T], Nodes) ->
           ?UTIL_MODULE:node_name(Address)
       end
   end,
-  extract_nodes(T, lists:merge(Nodes, [NodeName])).
+  NodeName.
 
 -spec maybe_add_acl(QArgs :: list()) -> list().
 maybe_add_acl(List) ->