query_cache: Only allow iteration over MAX_CACHED_ITEMS.

Items at the end of the query are almost guaranteed to be a huge jump
from those in the non-pruned part of the query.  To avoid confusion,
we'll cut off the query at MAX_CACHED_ITEMS which is still imperfect,
but better.

r2/r2/models/query_cache.py

@@ -108,7 +108,7 @@ class _CachedQueryBase(object):
     def __iter__(self):
         self.fetch()
 
-        for x in self.data:
+        for x in self.data[:MAX_CACHED_ITEMS]:
             yield x[0]