Move the code to clear RAM tables to rabbit_table

[Why]
Because it is specific to Mnesia and the logic has nothing to do in the
generic `rabbit` module.

[How]
A new `rabbit_table:maybe_clear_ram_only_tables/0` is introduced.

Author: dumbbell

deps/rabbit/src/rabbit.erl

@@ -1038,10 +1038,7 @@ prep_stop(State) ->
 
 stop(State) ->
     ok = rabbit_alarm:stop(),
-    ok = case rabbit_db_cluster:is_clustered() of
-             true  -> ok;
-             false -> rabbit_table:clear_ram_only_tables()
-         end,
+    ok = rabbit_table:maybe_clear_ram_only_tables(),
     case State of
         [] -> rabbit_prelaunch:set_stop_reason(normal);
         _  -> rabbit_prelaunch:set_stop_reason(State)

deps/rabbit/src/rabbit_table.erl

@@ -11,8 +11,9 @@
     create/0, create/2, ensure_local_copies/1, ensure_table_copy/3,
     wait_for_replicated/1, wait/1, wait/2,
     force_load/0, is_present/0, is_empty/0, needs_default_data/0,
-    check_schema_integrity/1, clear_ram_only_tables/0, retry_timeout/0,
-    wait_for_replicated/0]).
+    check_schema_integrity/1,
+    clear_ram_only_tables/0, maybe_clear_ram_only_tables/0,
+    retry_timeout/0, wait_for_replicated/0]).
 
 %% for testing purposes
 -export([definitions/0]).
@@ -191,6 +192,12 @@ clear_ram_only_tables() ->
       end, names()),
     ok.
 
+maybe_clear_ram_only_tables() ->
+    ok = case rabbit_db_cluster:is_clustered() of
+             true  -> ok;
+             false -> clear_ram_only_tables()
+         end.
+
 %% The sequence in which we delete the schema and then the other
 %% tables is important: if we delete the schema first when moving to
 %% RAM mnesia will loudly complain since it doesn't make much sense to