fix: handle possible unhandled promise rejection with autopipelining+cluster (#1467)
fix: handle possible unhandled promise rejection with autopipelining+cluster (#1467)

Deliberately add `asCallback` on the **same** Promise instance being returned,
not on a different Promise instance that resolves to the same thing.

Avoid this error:

```javascript
process.on('unhandledRejection', (reason) => console.log('unhandledRejection', reason));
const x = Promise.reject(new Error());
x.catch((e) => console.log('caught x', e));

const causesUnhandledRejection = Promise.resolve().then(() => x);
```

Related to #1466
luin/ioredis,luin/ioredis,luin/ioredis