I've been testing a scenario where processing the HTTP requests took longer enough for SIP 200 retransmissions to be triggered. In that case, `reply_received()` checks if the message was marked as suspended, but that works only if the onreply route calls `t_suspend()`. The retransmission was dropped instead, so I had to look into the transaction's T_ASYNC_SUSPENDED flag. The latest commit solves this, and should be safe for normal usages. I need to test more edge cases, will update.