Refactor promisify() code by using catering module

[ralphtheninja]

Closes #699

[vweevers]

Could be considered a breaking change because we did if (!callback) here, while catering does if (callback === undefined). The latter matches documented behavior though so I think it's OK. The docs never suggested we support e.g. await db.get('foo', null) the same as await db.get('foo').

[ralphtheninja]

Could be considered a breaking change because we did if (!callback) here, while catering does if (callback === undefined). The latter matches documented behavior though so I think it's OK. The docs never suggested we support e.g. await db.get('foo', null) the same as await db.get('foo').

Good catch. I didn't think that far. My OCD just told me to replace the code with your tested module :)