Async / Await Support

[countable]

I made these changes to work with mongojs in codebases that are async/await heavy (like puppeteer tests). Eliminating callbacks also seems true to the original design statement of mongojs, to be as similar as possible to the mongodb repl. There's a reasonable amount of new test coverage and all old tests pass. However, I realize the code changes are substantial. I'm not certain if you (maintainers) want to include this feature in mongojs due to its large scope. If you do, please consider making a major version bump to alert users that some things may behave a little different.

At a minimum, the library will use a bit more memory due to creating Promise objects whenever an API is called without a callback in existing code, as this absence of a callback is what signals the creation of a Promise now. This should be harmless most of the time but it's hard to know how it might interact with what's out there. Cheers, and thanks for a great little mongodb lib over the years!

[saintedlama]

There are alternatives like mongoist out there that support async/await out of the box