Implement a way to look up a key from the index
periodically-until-expired handles its own expiry now.

We check the high watermark time of the most recent event to determine
whether or not to continue running. Subject to a race condition, but
shouldn't be an issue for normal use. Closes #117.
Add some type hints.
Merge pull request #211 from banjiewen/master

Add some type hints
Merge pull request #210 from rrees/rr-kv-index-lookup

Lookup an event directly from the index