fix graceful restart and add unit test

tests/graceful_server.py

@@ -0,0 +1,13 @@
+#!/usr/bin/python
+
+import socket
+
+
+if __name__ == '__main__':
+    s = socket.socket()
+    s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+    s.bind(('127.0.0.1', 8001))
+    s.listen(1024)
+    c = None
+    while True:
+        c = s.accept()