Optimizations

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_ids 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.