Add iterator(options) and return [Symbol.asyncIterator]

I’d like to propose adding a method to levelup that would allow direct iteration of the underlying store.iterator. This would negate the need for consumers to convert the ReadStream to an Iterable.

from a usage perspective, something along the lines of:

async function main(){
  let db = levelup(encode(leveldown()));

  await db.put("foo", "hello");
  await db.put("bar", "world");

  let search = db.read( { leq: "bar", limit: 10 });

  for await (let data of search){
    let key = data.key;
    let value = data.value;
  }
}

not a huge change to implement this:

LevelUP.prototype.read= function (options) {
  options = extend({ keys: true, values: true }, options)
  if (typeof options.limit !== 'number') { options.limit = -1 }
  return new LevelAsyncIterator(this.db.iterator(options), options)
}

where LevelAsyncIterator can be the stream iterator butchered to remove the Stream aspects and simply implement [Symbol.asyncIterator] instead - along with ability to end (clean) the underlying store.iterator

If people are happy with this proposal I can make the PRs and a new level-iterator repo, based off level-stream-iterator?

I’ll do it as a plugin to levelup due to Symbol.asyncIterator being still stage-3.

[MeirionHughes]

Looks like async iteration is going stable - it is shipped in V8 - so hopefully we’ll see it without flags in node 9 https://github.com/tc39/proposal-async-iteration/issues/63#issuecomment-330978069

[vweevers]

Breaking this down:

• Add levelup#iterator: #586
• Add iterator#seek: Level/abstract-leveldown#234
• Implement multiget using levelup#iterator and iterator#seek (in userland to start) (#491)
• Add [Symbol.asyncIterator] to abstract iterator: Level/abstract-leveldown#235
• Add [Symbol.asyncIterator] to streams: will land soon in readable-stream.