rabbit_definitions: Import topic permissions after exchanges

[Why]
Topic permissions depend on an exchange, in addition to a user and a
vhost like other permissions.

This fixes a bug where an exchange imported after a topic permission
that depends on it caused the following crash when Khepri is used:

    {case_clause,{error,{khepri,mismatching_node,
                                #{node_name => <<"exchange_name">>,
                                  node_props => #{payload_version => 1},
                                  node_path =>
                                      [rabbitmq,vhosts,<<"/">>,exchanges,
                                       <<"exchange_name">>],
                                  condition => {if_node_exists,false},
                                  node_is_target => true}}}}

The crash comes from the fact that the exchange code expect to either
create the tree node in Khepri for that exchange, or there is an
existing tree node holding an exchange tree node. Here, there was a tree
node created implicitly when the topic permission was stored, but that
tree node didn't have an exchange record (because the exchange was not
imported yet).

[How]
We simply swap the import of topic permissions and exchanges.

Author: dumbbell

deps/rabbit/src/rabbit_definitions.erl

@@ -508,9 +508,9 @@ apply_defs(Map, ActingUser, SuccessFun) when is_function(SuccessFun) ->
         concurrent_for_all(vhosts,             ActingUser, Map, fun add_vhost/2),
         validate_limits(Map),
         concurrent_for_all(permissions,        ActingUser, Map, fun add_permission/2),
-        concurrent_for_all(topic_permissions,  ActingUser, Map, fun add_topic_permission/2),
 
         concurrent_for_all(exchanges,          ActingUser, Map, fun add_exchange/2),
+        concurrent_for_all(topic_permissions,  ActingUser, Map, fun add_topic_permission/2),
 
         sequential_for_all(global_parameters,  ActingUser, Map, fun add_global_parameter/2),
         %% importing policies concurrently can be unsafe as queues will be getting