Add async iterator interface to cursors

[mxstbr]

See #370, this patch adds a cursor.asyncIterator method which returns an async iterator:

const feed = yield r.db().table().changes().run();

const iterator = feed.asyncIterator();

for await (var change of iterator) {
  console.log(change);
}
console.log('Done!');

I added iterall as a dependency since I need to use it in the tests anyway, but if you want me to I can move it to the devDependencies and inline the $$asyncIterator polyfill snippet. (it's like 3 lines)

[mxstbr]

Wercker failed even though all tests passed 🤔

[sagivf]

@mxstbr What other errors might occur? why stop the iterations for only the above errors?

[mxstbr]

Good question, I copied that from the other places where self._next() is called. Not sure why it's that specific, honestly!

[sagivf]

I suppose it makes sense, since any other errors you might want to ignore where as these don't make sense to continue.

This does raise a question though. With callbacks you get the error and can ignore or handle it.
In this case you wont get it. maybe it should be a part of the return iter.return(err).

This also makes me really wonder what errors are possible... I'll ask around.

[segphault]

I came here to do this and was pleasantly surprised to find a PR already open. Now that async iterators are supported natively in node, this would definitely be nice to have, ideally without polyfill. Would it be possible to use the [Symbol.asyncIterator] protocol so that it's possible to pass the cursor itself directly into the for-await-of loop?

[mxstbr]

Would it be possible to use the [Symbol.asyncIterator] protocol so that it's possible to pass the cursor itself directly into the for-await-of loop?

Already possible!

[thelinuxlich]

@neumino ?

[thelinuxlich]

merged in https://github.com/RebirthDB/rebirthdb-js