Make db argument of iterator and chained batch required

[vweevers]

So that iterators and chained batches will always have a db property (or _db, normalizing that is a second question). This prevents garbage collection of db.

https://github.com/Level/abstract-leveldown/blob/7b079c5510f0ae354ae9503849ac7d9a0aa01551/abstract-iterator.js#L1-L2

https://github.com/Level/abstract-leveldown/blob/7b079c5510f0ae354ae9503849ac7d9a0aa01551/abstract-chained-batch.js#L1-L2

---

• [x] iterator
  • [x] make it required
  • [x] rename to .db (already the case)
  • [x] document
• [x] chained batch:
  • [x] make it required (#257)
  • [x] rename to .db
  • [x] document

[vweevers]

In addition, if _db is guaranteed to be there, we no longer need these "proxy" methods:

https://github.com/Level/abstract-leveldown/blob/7b079c5510f0ae354ae9503849ac7d9a0aa01551/abstract-chained-batch.js#L7-L13

[vweevers]

Should it be iterator.db (public) or iterator._db?

[ralphtheninja]

Should it be iterator.db (public) or iterator._db?

I have no strong opinion. If public it should be documented I guess.

[vweevers]

Let's go with public + docs. No particular reason.