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: I49bfd9673abc7602b27dc48b8b490daaded2882c
Closes-Bug: #1863021
(cherry picked from commit ec7a5aa)

neutron/common/eventlet_utils.py

@@ -37,3 +37,9 @@ def monkey_patch():
         # fail on Windows when using pipes due to missing non-blocking IO
         # support.
         eventlet.monkey_patch(os=False)
+    # 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
+    import threading  # noqa
+    orig_threading.current_thread.__globals__['_active'] = threading._active