Fix bug in in-memory data cache

Previously, if we didn't find a key, we'd empty the entire cache, making it essentially a single-key cache. We skip clearing now, although this does mean we won't GC expired entries (not a problem for tests, though)

data/cache/impl.py

@@ -40,7 +40,7 @@ class NoopDataModelCache(DataModelCache):
 class InMemoryDataModelCache(DataModelCache):
   """ Implementation of the data model cache backed by an in-memory dictionary. """
   def __init__(self):
-    self.cache = ExpiresDict(rebuilder=lambda: {})
+    self.cache = ExpiresDict()
 
   def retrieve(self, cache_key, loader, should_cache=is_not_none):
     not_found = [None]