Make get_state_groups_from_groups faster.

Most of the time was spent copying a dict to filter out sentinel values
that indicated that keys did not exist in the dict. The sentinel values
were added to ensure that we cached the non-existence of keys.

By updating DictionaryCache to keep track of which keys were known to
not exist itself we can remove a dictionary copy.

tests/util/test_dict_cache.py

@@ -28,7 +28,7 @@ class DictCacheTestCase(unittest.TestCase):
         key = "test_simple_cache_hit_full"
 
         v = self.cache.get(key)
-        self.assertEqual((False, {}), v)
+        self.assertEqual((False, set(), {}), v)
 
         seq = self.cache.sequence
         test_value = {"test": "test_simple_cache_hit_full"}