Add server-side flag-based thread stopping with socket timeouts

shared/exceptions.py

@@ -1,5 +1,11 @@
 class TCPChatException(BaseException):
     pass
 
+
 class DataReceptionException(TCPChatException):
     pass
+
+
+class StopException(TCPChatException):
+    """An exception that occurs when a thread finds a stop flag."""
+    pass