net.tcp: server should be in shut-down state before start

The .configure and .tick events can be called before the server is running. If
an addon calls ctx.master.shutdown() at this point, the server hangs forever
waiting on __is_shutdown_set. This patch fixes this by starting off with it
already set.

mitmproxy/net/tcp.py

@@ -847,6 +847,7 @@ class TCPServer:
     def __init__(self, address):
         self.address = address
         self.__is_shut_down = threading.Event()
+        self.__is_shut_down.set()
         self.__shutdown_request = False
 
         if self.address == 'localhost':