rabbit_feature_flags: Trap exit signal in the controller

[Why]
We need to do this for the `terminate/3` to be called. Without this, the
process exits without calling it.

(cherry picked from commit a472982)

Author: dumbbell

deps/rabbit/src/rabbit_ff_controller.erl

@@ -168,6 +168,7 @@ callback_mode() ->
     state_functions.
 
 init(_Args) ->
+    process_flag(trap_exit, true),
     {ok, standing_by, none}.
 
 standing_by(