Meteor 3.0 compatible API

[jankapunkt]

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