[feature request] Consideration of using async / await

## For feature request

[The native promise after node v10 is fast enough](https://github.com/petkaantonov/bluebird#note
).
Since `async` / `await` is supported in node v7 or later, it can be used when support of node v6 LTS (maintenance) is turned off.
Using `async` / `await` tends to make the code simple, and sequential asynchronous is faster than `bluebird` on [the `bluebird` side benchmark](https://github.com/petkaantonov/bluebird/tree/v3.5.2/benchmark) (in the case of parallel, now `bluebird` is faster).