Revert "Rely on close() to close sockets and stop using shutdown()"

This reverts commit 64e4f18dd09de90335c663b07fc0933646355665
because it broke the CI tests.

kmip/services/kmip_client.py

@@ -301,9 +301,10 @@ class KMIPProxy(object):
         self.close()
 
     def close(self):
-        # Close the socket.
+        # Shutdown and close the socket.
         if self.socket:
             try:
+                self.socket.shutdown(socket.SHUT_RDWR)
                 self.socket.close()
             except (OSError, socket.error):
                 # Can be thrown if the socket is not actually connected to

kmip/services/server/server.py

@@ -362,11 +362,12 @@ class KmipServer(object):
 
         self._logger.info("Shutting down server socket handler.")
         try:
+            self._socket.shutdown(socket.SHUT_RDWR)
             self._socket.close()
         except Exception as e:
             self._logger.exception(e)
             raise exceptions.NetworkingError(
-                "Server failed to close socket handler."
+                "Server failed to shutdown socket handler."
             )
 
         if hasattr(self, "policy_monitor"):

kmip/services/server/session.py

@@ -113,6 +113,7 @@ class KmipSession(threading.Thread):
                     self._logger.info("Failure handling message loop")
                     self._logger.exception(e)
 
+        self._connection.shutdown(socket.SHUT_RDWR)
         self._connection.close()
         self._logger.info("Stopping session: {0}".format(self.name))