Monkey patch original current_thread _active

Monkey patch the original current_thread to use the up-to-date _active
global variable. This solution is based on that documented at:
eventlet/eventlet#592

Change-Id: I5b00ee328f83cec8375ad1538be3a16059af08a3
Closes-Bug: #1863021
(cherry picked from commit 0b78dad)

networking_sfc/services/sfc/agent/__init__.py

@@ -1,2 +1,8 @@
 from neutron.common import eventlet_utils
 eventlet_utils.monkey_patch()
+# Monkey patch the original current_thread to use the up-to-date _active
+# global variable. See https://bugs.launchpad.net/bugs/1863021 and
+# https://github.com/eventlet/eventlet/issues/592
+import __original_module_threading as orig_threading  # noqa pylint: disable=wrong-import-position
+import threading  # noqa pylint: disable=wrong-import-position,wrong-import-order
+orig_threading.current_thread.__globals__['_active'] = threading._active