- Close() waits for eviction to complete before continuing, avoiding a
  "closed database" error
- Use CreateTemp() instead of +.tmp to avoid errors when concurrent
  writes occur to the same object
- Deal with another race when a file is evicted between Stat() and TTL
  loading from the database.

This is useful for backends like, eg. S3, where it's too expensive to
collect stats.

Retaining a reference to the internal buffer is not safe.