In #50 I made an early start with this, but I made two mistakes:

1. Putting the signal in per-method options, e.g. `next({ signal })`,
   rather than in constructor options, i.e. `iterator({ signal })`.
   The former would not be accessible in a `for await...of` and
   come with a performance penalty.
2. Thinking it could replace `abortOnClose` (#21), which is a
   separate issue. I'll leave it at that, because it only matters
   for `many-level` and can be solved later.

Anyway, this PR enables the following:

```
const abortController = new AbortController()
const signal = abortController.signal

// Will result in 'aborted' log
abortController.abort()

try {
  for await (const entry of db.iterator({ signal })) {
    console.log(entry)
  }
} catch (err) {
  if (err.code === 'LEVEL_ABORTED') {
    console.log('aborted')
  }
}
```

It also fixes small bugs in the `open()` and `close()` methods of
multiple classes, as follow-up for #50.