Add delays to SM buffer

It is rerouted when no session was resumed and the resume_timeout has passed.
This resulted in suprising messages, sometimes after a long time.

src/stream_management/mod_stream_management.erl

@@ -386,14 +386,28 @@ maybe_handle_stream_mgmt_reroute(Acc, _StateData, _HostType, _Reason, {error, no
 handle_user_terminate(#sm_state{counter_in = H} = SmState, StateData, HostType) ->
     Sid = mongoose_c2s:get_sid(StateData),
     do_remove_smid(HostType, Sid, H),
-    reroute_buffer(StateData, SmState),
+    FromServer = mongoose_c2s:get_lserver(StateData),
+    NewState = add_delay_elements_to_buffer(SmState, FromServer),
+    reroute_buffer(StateData, NewState),
     SmState#sm_state{buffer = [], buffer_size = 0}.
 
 reroute_buffer(StateData, #sm_state{buffer = Buffer, peer = {gen_statem, {Pid, _}}}) ->
     mongoose_c2s:reroute_buffer_to_pid(StateData, Pid, Buffer);
 reroute_buffer(StateData, #sm_state{buffer = Buffer}) ->
     mongoose_c2s:reroute_buffer(StateData, Buffer).
 
+add_delay_elements_to_buffer(#sm_state{buffer = Buffer} = SmState, FromServer) ->
+    BufferWithDelays = [begin
+                            TS = mongoose_acc:timestamp(Acc),
+                            StanzaName = mongoose_acc:stanza_name(Acc),
+                            StanzaType = mongoose_acc:stanza_type(Acc),
+                            {From, To, El} = mongoose_acc:packet(Acc),
+                            ElWithDelay = maybe_add_timestamp(El, StanzaName, StanzaType, TS, FromServer),
+                            AccParams = #{from_jid => From, to_jid => To, element => ElWithDelay},
+                            mongoose_acc:update_stanza(AccParams, Acc)
+                        end || Acc <- Buffer],
+    SmState#sm_state{buffer = BufferWithDelays}.
+
 -spec terminate(term(), c2s_state(), mongoose_c2s:data()) -> term().
 terminate(Reason, C2SState, StateData) ->
     ?LOG_DEBUG(#{what => stream_mgmt_statem_terminate, reason => Reason,