Use native Symbol.asyncIterator if available about levelup-async-iterator

[MeirionHughes]

I think the end-goal would be to add this to the levelup / leveldown implementations eventually.

Node is supposedly getting async-iterator support on streams - so Level guys may well not add direct access to the iterator from level-up and tell you to use the stream instead.

[huan]

Oh, sorry I just saw this issue. That's what I purposed in #2 because I'm just writing a wrapper of levelup db to use the async iterator.

See: https://github.com/zixia/face-blinder/blob/38ba45f263cc5cd892d034f8b115b470ed30bf0e/src/store.ts#L114

[huan]

Do you want to include this feature in the official levelup package in the future?

I suggest so because after doing that, users will be very convenient to use the async iterator.