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.

django/db/migrations/state.py

@@ -1,4 +1,4 @@
-from django.core.apps.cache import BaseAppCache
+from django.core.apps.cache import AppCache
 from django.db import models
 from django.db.models.options import DEFAULT_NAMES, normalize_unique_together
 from django.utils import six
@@ -32,7 +32,7 @@ class ProjectState(object):
     def render(self):
         "Turns the project state into actual models in a new AppCache"
         if self.app_cache is None:
-            self.app_cache = BaseAppCache()
+            self.app_cache = AppCache()
             # We keep trying to render the models in a loop, ignoring invalid
             # base errors, until the size of the unrendered models doesn't
             # decrease by at least one, meaning there's a base dependency loop/