Avoid calling callbacks while holding locks

Instead of calling the callback while holding
a write lock (a recipe for deadlock) call the
callbacks after we pop all the values to avoid
any potential for deadlock.

Change-Id: Idb4275d8f07d60e2ed83c0987b6c7354a09c8a71

taskflow/engines/worker_based/cache.py

@@ -51,7 +51,8 @@ class Cache(object):
         with self._lock.write_lock():
             expired_values = [(k, v) for k, v in six.iteritems(self._data)
                               if v.expired]
-            for k, v in expired_values:
-                if on_expired_callback:
-                    on_expired_callback(v)
+            for (k, _v) in expired_values:
                 self._data.pop(k, None)
+        if on_expired_callback:
+            for (_k, v) in expired_values:
+                on_expired_callback(v)