Terminate() is called when Zeek shuts down. If WebSocket client threads
were blocked in QueueForProcessing() due to reaching queue limits, these
previously would not exit QueueForProcessing() and instead block
indefinitely, resulting in the ixwebsocket library blocking and its
garbage collection thread running at 100%. Not great.
Closing the onloop instance will unblock the WebSocket client threads
for a timely shutdown.
Closes#4420
