client: revert incorrect memory optimization

Since commit d2d77437 ("client: make the cache
tolerant to late invalidations when the entry is in the history queue"),
invalidated items became current again when they were moved to the history
queue, which was wrong for 2 reasons:
- only the last items of _oid_dict values may have next_tid=None,
- and for such items, they could be wrongly reused when caching the real
  current data.

neo/client/cache.py

@@ -121,7 +121,7 @@ class ClientCache(object):
             item.expire = self._time + self._life_time
         else:
             self._size -= len(item.data)
-            item.data = item.next_tid = None
+            item.data = None
             if self._history_size < self._max_history_size:
                 self._history_size += 1
             else: