Consider returning a promise from flush method in node tracker

[adatzer]

Is your feature request related to a problem? Please describe.
Currently gotEmitter's flush method is not async and returns void instead of a promise.
This makes it harder to use in environments where a promise is needed to ensure all async tasks complete before exiting (e.g. AWS Lambda async handlers).

Describe the solution you'd like
flush is async/returns a promise.

Describe alternatives you've considered
At the moment the way to ensure flush completes in such environments is to wrap the emitter in a promise, and configure it with a callback that resolves and rejects that promise.

[paulboocock]

I've often pondered if we should "promisify" the whole tracker...