[Meteor 3] Update package to async

[sebastianspiller]

Hello there!

I made some changes to make synced cron work with async. I also translated it to es6, which I will add in a future pr, so that we can discuss changes in smaller steps.

In tiny test, I still get errors with, so it is just a wip pr:

• exception - message Cannot await without a Fiber
• exception - message Meteor code must always run within a Fiber. Try wrapping callbacks that you pass to non-Meteor libraries with Meteor.bindEnvironment.

It would be nice, if you have a look, thank you.

Regards
Sebastian

[dallman2]

I am doing the same thing to allow my Meteor 3 app to run. Please merge these changes.