The backend does not serve expired but still present entries so to a
user they do not exist. When they put new data over such an entry their
expecation is that the value is overwritten, even if not explicitly
requested.
The SQLite storage backend implements expiration by hand and garbage
collection is done in `DoExpire`. This previously relied exclusively on
gets not running within `Storage::expire_interval` of the put, otherwise
we would potentially serve expired entries.
With this patch we explictly check that entries are not expired before
serving them so that the SQLite backend should never serve expired
entries.