check shutdown flag in worker loops so they don't lock up the exit process

rtu/threads.lua

@@ -121,7 +121,7 @@ function thread__comms(smem)
         -- thread loop
         while true do
             -- check for messages in the message queue
-            while comms_queue.ready() do
+            while comms_queue.ready() and not plc_state.shutdown do
                 local msg = comms_queue.pop()
 
                 if msg.qtype == mqueue.TYPE.COMMAND then