Some optimizations to speed up importing messages. Speed up is 2x, from 400 sec to 200 sec to import 10k messages, on local pod.

General idea is to:

* Use `set` instead of `list` when we check for existance of items.
  * This requires that `Item` is hashable, which is not. So I patched schema items with `__hash__` function and used `external_id`s as hashes, which should be unique anyway.
* Use `lru_cache` when querying item by id, so we don't have to wait for pod to respond.