Make BaseFileCache into an abstractbaseclass so it's a proper interface

Push the opening and closing of files into the _load and _dump methods
so that we don't invoke the slow codec machinery without reason.

lib/ansible/plugins/cache/pickle.py

@@ -30,13 +30,13 @@ class CacheModule(BaseFileCacheModule):
     """
     A caching module backed by pickle files.
     """
-    plugin_name = 'pickle'
-    read_mode = 'rb'
-    write_mode = 'wb'
-    encoding = None
 
-    def _load(self, f):
-        return pickle.load(f)
+    def _load(self, filepath):
+        # Pickle is a binary format
+        with open(filepath, 'rb') as f:
+            return pickle.load(f)
 
-    def _dump(self, value, f):
-        pickle.dump(value, f)
+    def _dump(self, value, filepath):
+        with open(filepath, 'wb') as f:
+            # Use pickle protocol 2 which is compatible with Python 2.3+.
+            pickle.dump(value, f, protocol=2)