Add the concept of a data model cache, for caching of Namedtuple objects from the data model

Will be used to cache blobs, thus removing the need to hit the database in most blob requests

data/cache/cache_key.py

@@ -0,0 +1,8 @@
+from collections import namedtuple
+
+class CacheKey(namedtuple('CacheKey', ['key', 'expiration'])):
+  """ Defines a key into the data model cache. """
+  pass
+
+def for_repository_blob(namespace_name, repo_name, digest):
+  return CacheKey('repository_blob:%s:%s:%s' % (namespace_name, repo_name, digest), '60s')