fix(emqx_connection): connection OOM kill immediately

Prior to this change, connection process' OOM kill is a loopback
message, which results in a delayed kill when message queue length
is large.
In this change, the loopback message is changed to an exit signal.

src/emqx_connection.erl

@@ -684,9 +684,11 @@ check_oom(State = #state{channel = Channel}) ->
     Zone = emqx_channel:info(zone, Channel),
     OomPolicy = emqx_zone:oom_policy(Zone),
     case ?ENABLED(OomPolicy) andalso emqx_misc:check_oom(OomPolicy) of
-        Shutdown = {shutdown, _Reason} ->
+        {shutdown, Reason} ->
-            erlang:send(self(), Shutdown);
+            %% triggers terminate/2 callback immediately
-        _Other -> ok
+            erlang:exit({shutdown, Reason});
+        _Other ->
+            ok
     end,
     State.