rabbit_deprecated_features: Improve reliability of the classic_queue_mirroring callback

[Why]
The callback wants to query the `rabbit_runtime_parameters` Mnesia table
to see if there HA policies configured. However this table may exist but
may be unavailable. This is the case in a cluster if the node running
the callback has to wait for another cluster member before Mnesia tables
can be queried or updated.

[How]
Once we verified the `rabbit_runtime_parameters` Mnesia table exists, we
wait for its availability before we query it. `rabbit_table:wait/2` will
throw an exception if the wait times out.

In particular this avoids an infinite loop in
`mnesia_to_khepri:handle_fallback()` because it relies on the
availability of the table too to determine if the table is being
migrated or not.

Author: dumbbell

deps/rabbit/src/rabbit_mirror_queue_misc.erl

@@ -962,8 +962,16 @@ are_cmqs_used(_) ->
             RuntimeParamsReady = lists:member(
                                    rabbit_runtime_parameters, AllTables),
             case RuntimeParamsReady of
-                true  -> are_cmqs_used1();
-                false -> false
+                true ->
+                    %% We also wait for the table because it could exist but
+                    %% may be unavailable. For instance, Mnesia needs another
+                    %% replica on another node before it considers it to be
+                    %% available.
+                    rabbit_table:wait(
+                      [rabbit_runtime_parameters], _Retry = true),
+                    are_cmqs_used1();
+                false ->
+                    false
             end
     end.