Meteor 3.0 compatible API

We need to update this to become compatible with Meteor's async API in order to enable some level of isomorphism.

The API changes for Mongo Collection and Cursors are described here: https://github.com/meteor/meteor/blob/d5c3b2eeafd0ad78ee7e2553f3f269c5c2a2e2a9/docs/generators/changelog/versions/3.0.md#L5-L17