dht: expire announcement tokens after 30 minutes

dht::Database was exhibiting unbounded memory consumption due to
generating and remembering announcement tokens without ever expiring
and forgetting them. This commit augments the existing
dht::Database::expire(bt::Timestamp) function so that it erases tokens
that are more than MAX_ITEM_AGE (30 minutes) old. Also, it deletes and
erases DBItemList instances when they have been emptied by expirations.