Add plugable locking backend support. Add threadsafe file lock backend.

mayan/apps/lock_manager/backends/model_lock.py

@@ -0,0 +1,10 @@
+from __future__ import unicode_literals
+
+from django.apps import apps
+
+
+class ModelLock(object):
+    @classmethod
+    def acquire_lock(cls, name, timeout=None):
+        Lock = apps.get_model(app_label='lock_manager', model_name='Lock')
+        return Lock.objects.acquire_lock(name=name, timeout=timeout)