[WIP] Promisify client-facing methods

[alecgibson]

This change makes client-facing methods return a Promise when no callback is provided.

Motivation

• As a consumer of ShareDB using modern JavaScript with Promises and async/await, I'm bored of using callbacks, which are inconsistent with the rest of my codebase
• As a contributor to ShareDB, I'm bored of tests with deeply nested callbacks (and other maintenance burdens imposed by callbacks)

Work so far

This is a work-in-progress pull request in order to get opinions on how people feel about this as a feature; and how it should be implemented.

So far, I have added a promisify method to util, which has the following signature:

util.promisify = function(callback): { promise, callback }

If a callback is provided, then promise is undefined, and the returned callback is the same as the provided callback. This is to avoid clients inadvertently creating logic forks, where code follows both a callback and a Promise.

If callback is not a function, then the returned callback is a function that will resolve or reject the returned promise.

Also, for exiting functions early, callback() will return the promise, so you can write things like: if (condition) return callback();.

Discussion

Through this work I've discovered at least two points for discussion:

• Implementing this in Doc as it currently is means that emit is basically never called, because callback will now always be defined. I don't know enough about how people use these events, but we could potentially have consumers configure ShareDB to enable promises? If promises are not enabled, then util.promisify will always return the same callback as provided (even if it's falsy).
• The signature for a Query callback is callback(error, results, extra), but a Promise can only resolve a single argument. Should we combine this into { results, extra } for the purposes of a Promise?
• Do we bother including a polyfill for Promise, or just not bother promisifying if Promise is undefined? (And if we avoid a polyfill, how do I get the linter to calm down without targeting ES6?)

[coveralls]

Coverage decreased (-82.2%) to 14.336% when pulling 2b1f966 on alecgibson:promises into 762e05d on share:master.

[alecgibson]

Closing this, as apparently @nateps is not keen on the idea of Promises in this library.