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

mayan/apps/checkouts/tasks.py

@@ -5,6 +5,7 @@ import logging
 from django.apps import apps
 
 from lock_manager import LockError
+from lock_manager.runtime import locking_backend
 from mayan.celery import app
 
 from .literals import CHECKOUT_EXPIRATION_LOCK_EXPIRE
@@ -17,15 +18,12 @@ def task_check_expired_check_outs():
     DocumentCheckout = apps.get_model(
         app_label='checkouts', model_name='DocumentCheckout'
     )
-    Lock = apps.get_model(
-        app_label='lock_manager', model_name='Lock'
-    )
 
     logger.debug('executing...')
     lock_id = 'task_expired_check_outs'
     try:
         logger.debug('trying to acquire lock: %s', lock_id)
-        lock = Lock.objects.acquire_lock(
+        lock = locking_backend.acquire_lock(
             name=lock_id, timeout=CHECKOUT_EXPIRATION_LOCK_EXPIRE
         )
         logger.debug('acquired lock: %s', lock_id)