Iterator interface leaks files

A simple statement like 

``` ruby
key, value = db.each(reversed: true).first
```

will leak an iterator and therefore open file descriptors. (even if the db is closed) Doing enough of these and your Ruby process crashes because it exhausts your file ulimit. 

We would need an explicit iterator interface or at the very least a #close method on the iterator so that the open files can be closed. 
