[feature request] Consideration of using async / await

For feature request

The native promise after node v10 is fast enough. 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 (in the case of parallel, now bluebird is faster).