Rewrite expiry tests.
Expire events if their age is equal to the expiry time.
Add basic benchmark script for Index.
Add deadlines to index.
Test that removing an event removes the deadline.
Refactor Index.
Refactor deadline and index key to Event class.
Make expire use deadlines to find expired events.

This ensures we do not have to iterate over all the events in the
index to find events past their deadline.
Tweak benchmark script.