Meteor 3.0 compatibility

[jankapunkt]

This is now the updated migration base-branch, exceeding #740 / #738
This means all of the following tasks should be done in an own PR, pointing to this one's branch (migrate/3.0)

There has to be some preliminaries in order to be as non-breaking as possible:

• update coverage for all the missing files (run coverage script in testapp to see what's missing)
• get rid of clone dependency, replace with more native+distinct solution
• lint-fix according to our community standards

The actual migration will then require the following:

• make all computations async compatible
  • clean
  • validate
  • validator
  • ... what else?
• make all Tracker-dependent functions compatible with async Tracker
  • labels
  • label
  • messages
• ... what else?

[jankapunkt]

I come to the point to realize there is no way to support both sync and async computations. Since any method call will require async this bubbles down to all functions involved in the validation chain.

It's also nearly impossible to break up the existing syn structures and make them partially async using conditional return of Promises, where applicable.

It works for the labels but it does not work at all for doValidation which is basically the core validation routine, without making it all async.

I therefore will go down the path of making it fully async. Any objections please add here.