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)

util/expiresdict.py

@@ -17,9 +17,9 @@ class ExpiresEntry(object):
 class ExpiresDict(object):
   """ ExpiresDict defines a dictionary-like class whose keys have expiration. The rebuilder is
       a function that returns the full contents of the cached dictionary as a dict of the keys
-      and whose values are TTLEntry's.
+      and whose values are TTLEntry's. If the rebuilder is None, then no rebuilding is performed.
   """
-  def __init__(self, rebuilder):
+  def __init__(self, rebuilder=None):
     self._rebuilder = rebuilder
     self._items = {}
 
@@ -49,6 +49,9 @@ class ExpiresDict(object):
     return self.get(key) is not None
 
   def _rebuild(self):
+    if self._rebuilder is None:
+      return self._items
+
     items = self._rebuilder()
     self._items = items
     return items