Deborgified the app cache.

Improved Andrew's hack to create temporary app caches to handle
migrations. Now the main app cache has a "master" flag set to True
(which is a non-default keyword argument, thus unlikely to be used by
mistake). Other app cache instances have "master" set to False.

The only sanctioned way to access the app cache is by importing
django.core.apps.app_cache.

If you were instanciating an app cache and relying on the Borg pattern,
you'll have to refactor your code.

tests/validation/test_unique.py

@@ -3,7 +3,7 @@ from __future__ import unicode_literals
 import datetime
 import unittest
 
-from django.core.apps.cache import BaseAppCache
+from django.core.apps.cache import AppCache
 from django.core.exceptions import ValidationError
 from django.db import models
 from django.test import TestCase
@@ -58,7 +58,7 @@ class GetUniqueCheckTests(unittest.TestCase):
 
                 Meta = type(str('Meta'), (), {
                     'unique_together': unique_together,
-                    'app_cache': BaseAppCache()
+                    'app_cache': AppCache()
                 })
 
             checks, _ = M()._get_unique_checks()