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/test/test_cache.py

@@ -0,0 +1,16 @@
+import pytest
+
+from data.cache import InMemoryDataModelCache, NoopDataModelCache
+from data.cache.cache_key import CacheKey
+
+@pytest.mark.parametrize('cache_type', [
+  (NoopDataModelCache),
+  (InMemoryDataModelCache),
+])
+def test_caching(cache_type):
+  key = CacheKey('foo', '60m')
+  cache = cache_type()
+
+  # Perform two retrievals, and make sure both return.
+  assert cache.retrieve(key, lambda: 1234) == 1234
+  assert cache.retrieve(key, lambda: 1234) == 1234